Worldbuilding

I'm only claiming layperson knowledge. I do not have a Biology or Physics professional background. I'm going to go less traditional here.

I'm going to say in this fictional world, the lightning strikes through the island. As in, a lightning bolt strikes from the high clouds to the island's surface, and then another bolt forming from the underside of the island striking down (potentially all the way to the ground). Since lightning in relativity is the equalization of electrical potentials between the clouds and the ground from moving masses of air, your flying island acts as a bit of a flying lightning rod. Its large enough (unlike a passenger jet) to act as a conductor for the ions of either charge.

Islands drift with the wind and bob up and down as the coral gets dehydrated from being above the clouds and some of it dies, reducing lifting gas production and making the island sink until it becomes healthier again.

When the island's altitude is low, the larger strike will occur on the top surface of the island. When the altitude is high, it will be a smaller strike on the surface, and much larger bolt extending from the underside of the island going down to the ground. Further, depending on the size of the bolt, and the exit point on the bottom, some coral will die. This can cause the island to list to one side until the coral is healed/replaced. You could even have perpendicular or temporarily upside down islands because of the uneven remaining distribution of coral polyps.

Another scenario: If an island is small enough, or a bolt of lightning large enough, near all (or all) coral dies in the lightning strike, and the flying island dies, crashing into the grown in a giant, loud, catastrophic event. Perhaps small colonies of coral polyps surviving a crash to the ground is also the the birthplace of future flying islands. The small amount of surviving coral is jammed deep underground in the island crash where it grows and reproduces over X period of time, eventually tearing a large amount of ground out with it as a new flying island joins the world.

You could even have characters weaponize this. Plant flying coral underneath the cities of your foes. Years/decades later, large sections of the city abruptly rip themselves out of the ground becoming islands and destroying large parts of the cities with it. This could be extended to societal laws preventing the import of coral into cities, with high penalties for doing so.

Lots of great possibilities for telling stories in this world:

• warfare - governments using them as the equivalent of battleships as they fly over enemy towns and strike from above.
• domesticating them for transportation
• selective breeding programs for specialized corals with greater growth rate, higher lightning resistance, or increased lift with smaller size
• religion surrounding the origins of the island or their reproduction
• scarcity - the story of the people that find the last floating island in the world

First thing that comes to mind: evolution lead to symbiotic union with another coral species that lives of stored electricity from lighting strikes, thereby protecting the other coral type.

Regarding the effect of a lighting strike: the biggest problem would probably be the change in electrical charge on the island as a whole. But not for the island travelers while on the island, but each time they interact with somebody/something from outside the island. Because the difference in static electrical charge would lead to an equilibrium between the two entities, i.e. a lighting strike. So every time a ship would land at the island, it would get zapped. Same if somebody from the island would get close to the ground.

Good things to future out since cloud to cloud lightning is significant and if the island is large it would likely be in the path often. Not an expert for for story telling, might make sense that it is handled relatively benignly. Maybe the coral is a poor path to follow or is very attractive, making the strikes really frequent and light, hardening the sides and creates a pocket of relatively low energy in the surrounding clouds.

Corals would probably be more conductive than ground itself, an evolutionary pressure might force it to either stay together, very closely, forming larger islands with pillar like structures to minimise impact, or create a specific organ that you might (maybe) excavate in your stories and use it as a proto battery

Depending on how big the island is, and how the gas is stored, I don't think it would drop like a rock, I think it would slowly drift down as gas escapes. This reminds me of Laputa from Gulliver's Travels. Perhaps residents could weaponize the islands by selectively reducing buoyancy to crush cities below.

I have a setup involving floating cities in the upper atmosphere of a gas giant, but instead of using a lifting gas they use Flanar pontoons.

Lightning would probably be bad for any organism but thankful, on whatever planet things your corals are on, the gas is an excellent conductor which safely directs the lighting around and though the coral, creating amazing light shows.

Sadly though, if one gets close enough to the ground and there's not enough gas production, errant strikes can happen, leading to at least one tragic event in the past - since that time, dwellers on any coral are diligent about watering their... Yard? And have either magical or physical parachutes if they're particularly well off. Larger colonies have taken to drilling a lightning rod though the whole comet (I dunno, sounded like a good name) but this has felled more than one, so settlers need to find really good specimen for that procedure - A strike of dry coral, due to the air gaps in the structure, can engulf an 80 sq. km island in under 8 hours, so any colony without active protection is more nomadic and less 'earthly possessions' than other tribes because when the bell rings, you go or you die.

Pyroanthropology is a passion of mine, so this was a fun thought experiment. I'd imagine if the civilization was industrialized enough, they'd be trying to harvest and utilize the power but I could also see them becoming a siege weapon, essentially a flaming shot you could drop on your enemies heads.

Edit: Forgot to add, due to the conductive nature of the gas, residents would need to stay inside during storms or go out wearing essentially gimp suits worth of rubber - so emergency services would look really funny.

This thread reminds me of the fifth Bobiverse novel: Not Til We Are Lost. The 'flying islands' in this story are enormous living beings, buoyed by stores of flammable gas. They live on a volcanically active planet, and only occasionally explode.