Got a Eweadn mechanical keyboard and I really like it. After months of use, however, this greenish teal/turquoise stuff accummulates on the keys I use the most. It also accumulates on the sides of the keys, and it doesn't clean off easily. It seems to originate between the circular cap and the rose-gold plated rims. What is?

I apologize if this isnt the correct community to post this in, i couldnt find a generic 'what is it' community.