I have had this problem for a while and thought that I may have figured out the problem. I was wrong. I remodeled my upstairs a few years back and did a total overhaul of my bathroom. I now have this green stuff (copper sulfate) collecting on my shower. At first I figured I had some kind of electrical potential across my pipes somewhere, but I have put jumpers across all kinds of fittings, water softener, hot water tank, etc. I don't have a "floating" ground. That can really screw you up, along with knocking you into next week. So I am at a loss as to what is causing this and how to correct it. I don't think my water is acidic, but I do have some external green crud on some of my plumbing fittings. That is probably due to flux on the outside.