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The nickel platting on the copper has worn off. The copper underneath is actually more conductive than the silver nickel platting, but the scratches on it do the opposite with contact and conductivity. Just use thermal paste, and spread it well, and you'll be fine.
If you rub harder, or try to polish it off, you'll just see yellow copper underneath.
Bro it literally doesn't matter
Probably something stucl in microabrasions, don’t think it can be removed without polishing.
Do you NEED it gone?
Also rule #5, your post will most likely get removed, so perhaps ask PCMR or similar subs.
You could try using a small microfiber cloth with alcohol (you did mention previously trying IPA, but avoid using acetone as it can affect the substrate around the IHS) and even pressure to get a consistent movement while cleaning. If you brush your fingernail gently across the surface, do you feel texture? If so, it's likely light abrasion and further cleaning won't help.
It's essentially cosmetic, you can apply new paste and carry on as normal. Paste is designed to fill those small imperfections to begin with, and you won't notice once you start using your system. :)