Start with running hot water on it and see if that works. If not, try a very gentle dab with a Q-tip. I use pure acetone to get big gunk off coins but only when I have to.
I wouldn't waste the time it'll take for that coin since that green stuff is corrosion and it's not going to come off. I've encountered similar corrosion on IKE Dollars and nothing works short of a brillo pad which just screws up the coin.
Yes, if you get enough of them. 30 million of them like the OP coin and you should be able to retire fairly comfortably.