Electrolysis will clean that up in no time...just find an old phone recharger you don't need, splice the lines (you can determine negative and positive with a cheap voltage meter at Home Depot) and then attach those wires to alligator clips (red (positive) and black (negative).
Find an old ratty metal spoon and a pint glass you don't care about - fill the glass with warm salt water, attach the negative clip to the top end of the handle of the spoon and the positive clip to the coin, then submerge the coin.
THEN PLUG INTO THE OUTLET...
You should start to see the coin fizz after about 5-10 seconds, if it doesn't just UNPLUG, and switch the alligator clips. Once the fizzing starts watch the gunk just fall off in droves, it's crazy, especially with silver coins.
Good luck!