Try the three stages of conservation,
First: soak it in distilled water to remove surface dirts and whatnot. Repeat 2 or 3 times or until water doesn't change color or lose clarity. Change water frequently so the stuff doesn't re-deposit.
Second: give it an acetone bath (remember only 100% pure). Do it for an hour or 2 at a time, make sure to rinse it with more distilled water after removing it, and change out the acetone. Repeat a couple times.
Third: if you proceed to this step post pics first so we can evaluate. I have found semi-key indians and sometimes verdi-care is needed to stop or slow the worst of the corrosion. Don't do this without consultation. Verdi-care can react oddly with burried coins, especially nickels, because of how they react with ground contamintes.