I am a chemist, not a plumber, but I do water quality testing as a sideline to my fulltime job doing protein synthesis. Copper turns green when it comes in contact with either a strong oxidizer (like bleach, not chlorine, since bleach is hypochlorite). My bet is that the water is more acidic than to be expected like a pH of 6 or lower. pH testing is one of the simplest to do, so I’d start there.
[Edited by Richard on 31 October 2000]