@eta, that looks a lot like dried adhesive. You could try a soak in acetone to remove it; however, the surface of the nickel may now be toned differently.
"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
Yes no prob. Give it a soak and then try to chip at it with a wooden toothpick (obviously you don't want to scratch the coin while you remove the glue). It would be interesting for you to post "after" pics to this thread if you decide to go forward with the removal process.
"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
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