marbles wrote:
Some copper water pipe solder fittings have a residue of green gunge
around them, due to the finished solder joints not being effectively
cleaned after soldering.after new CH installation around 5 years ago.

Resues of acid/active flux, I believe. The active flux contains
hydrochloric acid which removes the surface oxide layer &
'cleans'/pickles the pipe and fitting when heated, prior to the solder
melting & running.

I don't know what the green compound is, I'm sure a chemist will be
along shortly. If left inside a heating system, the acidic flux
residues can cause serious problems with corrosion.