I agree, it looks like glue or tape residue. Try a Q-tip soaked in acetone on that. The coin is pure nickel, you won't hurt it.
"Discovery follows discovery, each both raising and answering questions, each ending a long search, and each providing the new instruments for a new search." -- J. Robert Oppenheimer
Looks like someone may have tried to seal the coin in a protective coating, realized it wasn't working out, and tried to clean all the clear stuff off before getting fed up and tossing it in the change basket.
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