Verdigris is mildly toxic by the way. This metal, whatever it is, seems like a bad choice for keycap material. Might want to wash hands after use.

This is corrosion of the copper in the brass by oxygen. Clean with mild acid like vinegar.

As people have said it's likely corrosion. More specifically, verdigris. It's the same sort of corrosion you see on the nosebridge of glasses. It's caused by the oils on your skin reacting with the bare metal of the keycaps. There isn't really a solution to stop it, apart from putting a clear coat over the metal.

Checked the board online (https://www.eweadn.com/products/eweadn-v20-typewriter-keyboard)

$40 for a full keyboard with "electroplated" keycaps. Others are right that vinegar will clean them right off but I suspect there's not gonna be much of anything left after it goes in for a soak.

Others may have better ideas on what can be done to preserve them at this point, but for something like that which likely has a very thin coat of easily oxidized metal the best maintenance is gonna be preventative. IDK what the best option is but I'd consider hitting it with a super thin coat or two of a drying oil like tung oil upon receipt. Wipe each key down with an oily rag, then wipe it off with a dry rag like you made a mistake. Allow 1-2 weeks for the tung oil to dry (plasticize). Clean all keys and reapply oil at the first sign of oxidization on any keycap.

Looks like copper corrosion, I'm guessing its gold plated copper, and the oils/sweat in your fingers is corroding the copper underneight?

Definitely copper corrosion. Rose gold is an alloy of copper and gold, and since it's a mechanical keyboard, I doubt they're springing for super high quality stuff, so it's probably more copper than gold. As another said, white vinegar with a few tablespoons of salt should remove the corossion, but I'm not sure how the rest of the keycap will hold up.

To add to what others are saying:

You can clean them, but the corrosion will come back over time.

Looks like finger jam. You can tell it's not lung butter, or else it'd be yellow.