I'm confident that the green "stuff" has been caused by some chemical reaction.
Sort of unusual for nickel alloy coins but obviously not out of the realm of possibility.
IF it is the result of some chemical reaction then there is no way of removing it short of cleaning the coin.
BUT
There's cleaning and then there's "cleaning".
If it were mine and I was willing to chance I'd soak it in olive oil for a month or so.
Pure extra virgin olive oil.
It is very slightly acidic.
This treatment is normally used only on copper coins BUT because of the color I suspect the cause is the copper in the alloy.
And it will take a month or more; inspect it every two weeks.
If it won't be a hard hit on your wallet if you make the coin worse give olive oil a try.
And if things improve and you want to stop the process THEN use acetone to flush away the olive oil.