Green Lincoln In Mint Case

Something might have gotten into the case. Or something was on the surrounding plastic when it was put inside.
Or the case might have been cracked, and someone replaced the original with that one.

I'm probably not right on most of these, but I hope they help if I am right.

I bet water got inside or moisture
I found a old penny under a carpet in a boat that looked like that ( boat was in fresh water I dont know how long the penny was under the carpet but it was mostly green)

Those lenses were not air tight and I am not sure if they were sealed (current ones are not sealed at all, late 1970s and early 1980s were glued but not air tight). I can't tell what year it is from this photo. There does not appear to be any discoloration of the purple matte but that is where I would first look for contamination. It almost looks like the coin was put into the lens with the contamination already on it and eventually it worked its evil magic. I think the purple was used from about 1984 to 1992 or so. If the lens is not glued, you can open it carefully (I know that 1995 was not glued) and remove the coin to see if the matte looks like the source of the contaminant. You might want to do this just so the other coins aren't eventually damaged as well.