Question About Problem Coins?

As a general rule, DON'T CLEAN COINS. The main reason is stuff on coins sometimes, and usually is, already combined with the metal of the coin. By removing that stuff, you would be removing some of the coin itself. It is always confusing to people about cleaning stuff. For example cleaning your car is a must. And no matter how old your car is, still cleaning is OK. Contrary is in most instances old oil paintings are usually not only cleaned but some are what is called restored.
Old furniture should never be cleaned. Your teeth should always be cleaned.
It always appears to be so confusing.
One thing though is it is always OK to try using what is called Acetone to get rid of stuff like tape, glue, possible green stuff on coins.

Frostbite ... There are many threads on the CCF, pertaining to cleaning and conserving coins. Try doing a search.

Regarding the green stuff, verdigris, experienced copper collectors often use a thorn to chip away the residue as that is not abrasive enough to scratch the surface. Generally speaking, coin cleaning is a "bad" practice, primarily because most collectors don't go about it properly. Books have been written about this practice. Conserving coins is, IMHO, a necessary practice. A principal case on point is that many of our older coins have "done time" in sulfurized albums and mylar flips that contain PVC. Such storage leaves damaging residue on coins that, if left to rest will impair a coin's surface. Careful removal of these substances, a.k.a. "conservation" (skilled cleaning) will help preserve these coins. Always practice on an old "wheatie" first ... just not a key date!