Welcome to CoinTalk! I think you are talking about the strange splotch on the reverse... I would think it is just dirt that has accumulated and stayed there. Otherwise, it could be just the effects of it being left in a hot place and melting and forming those things. Usually, if the coin is from 1983 of after (made mainly of zinc), and it has those splotches, it is just from a reaction with oxygen and other chemicals. I call it zinc-rot... Not nice stuff.
