green?? Oh, I see! You have copper pipes. Pretty dangerous stuff - the green is Copper Oxide which can turn into Copper Sulfate. The latter is poisonous.
Nah, the green is just slightly fluorescent dye they put in the water used in district heating. Makes leak detection easier. It's completely non-toxic. Of course I wouldn't recommend drinking the water since it has been circulating in those pipes for quite a long time.
"God runs electromagnetics by wave theory on Monday, Wednesday, and Friday,
and the Devil runs them by quantum theory on Tuesday, Thursday, and
-Sir William Bragg