I'm not sure there is much that can be done. I would keep an eye on it to make sure it doesn't get much bigger/worse. I don't know if verdicare would help or not, if you can find it.
Probably a malachite seed crystal (a water droplet with halite minerals) landed on the coin to form various copper hydroxides, oxides, sulfates, and carbonates (verdigris). More info in the thread Homemade Verdigris Formulas made by @GSDykes
