On all my copper pipes in my basement I have green discoloration mostly around the joint is it harmful it will not come off and is on it all the pipes around 7 years old. I heard it can eat thru but there is nothing I can do about it.
It sounds like excess flux oxidizing the copper. If a wet rag won't take it off, it's probably an oil based flux. Try mineral spirits on a rag: that should get it off. I see it in a lot of houses, so I don't think its all that serious, but I could be wrong.