I agree with @jd and @cj that this is dried glue. You should be able to peel it off the surface with a sharp edge like a razor blade. Soaking in the right solvent for long enough would remove it also (acetone is a good start, but there are others). Regardless, this coin is a spender.
"If you climb a good tree, you get a push."
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz