So I've gone further afield. I've found other people reporting the same problem with other models of car, but one in the same area as I am. And a posited theory that it has something to do with ethanol.
links:
https://www.answers.com/Q/What_can_i..._above_gas_cap
https://community.cartalk.com/t/blac...gas-door/81002
Black mold around fuel door - PeachParts Mercedes ShopForum
Still guessing, but I'm wondering if I don't have mold in the gas tank.