Why is there green stuff on my heater pipe?
When copper oxidizes, it first turns dark brown (like a copper penny), and then if left undisturbed into a somewhat moldy-looking green. This is normal. If you notice the pipe starting to deteriorate around areas that have a lot of green, this may indicate a leak in the pipe that is introducing more water into the area, speeding oxidation.

Just keep an eye on it to ensure it isn't deteriorating. Usually, the layer of oxidation forms a good protection of the copper behind it against further oxidation, and in this way that green stuff is beneficial.