What does loads of green rusty textured stuff on copper mean?

[WaterTight]

Was shown an area previously hidden within a wall. Some of the copper pipes were clean as a whistle, others were covered in the green stuff. Some parts would have a dot of green here and there, others you couldn't see copper for the green, often covering sections a foot long. No drips or obvious leaks.

 

[Simon F]

well its corrosion for whatever reason.
damp, condensation, flux etc.