>- basic Don Croft zapper to my cheek which hurt
One of the flaws of this otherwise unit. Too much electrical current concentrated into a small area and the electrodes are too close together which limits the overall reach.
>- with black/greenish in the middle
Possibly normal oil residue combined with copper salts.
>- do the copper cents sometimes have this greenish colorization?
Copper combines with chlorine to make greenish copper chloride ( toxic ) or with sulfates to make bluish copper sulfate ( also toxic ).
Copper electrodes should always be large enough to handle the current, be far apart, and be covered with wet paper towels to keep the copper off of the skin and out of the body.