I never did figure out what the green stuff was but it was super hard and very encrusted on the coin. I figure it is maybe some kind of chemical reaction from being in the ground possibly up against another metal object maybe. Heck I don't know but I got it cleaned up some.
@bcuda , you have set up your membership so that no one can start a private conversation with you. I would message you individually but I can't. By the way, you did a nice job getting the green off.
It is called Metal Rescue and is a rust remover that is non acidic, no alkalis, and has no solvents. I already had this stuff so I figured why not try it in a little spot. I dabbed it on to the green stuff with a toothpick and let it sit for a few hours. Then I rubbed hard at the green crud with a wood toothpick and the crud had softened up and started coming off. I had to repeat this about 3 times or so to get it to where it is at now. Here is a pic of the stuff.
