Re: green surface corrosion on copper water pipe
If the green colored corrosion is located at points of many or all soldered connections then likely it's from the acid flux used during the soldering.--- very common occurrence.
Typically someone that's experienced with soldering plumbing will use a wet cloth to wipe off the excess flux immediately after soldering. Unfortunately many DIY'ers who don't know any better or lazy hack plumbers skip this step.
How can I remove it and keep it off?
As for cleaning --- there's a couple of ways that would work.
1 - heat the area enough to reactivate the residual acid flux and wipe off with a wet rag.
This can be a little tricky if you are unsure what you are doing. You don't want too much heat to loosen the solder and compromise the connection . Though having the lines filled with water makes it more difficult to heat the solder enough to loosen --- it still can happen.
2 - use emery cloth to sand off the corrosion along with the acid flux --- so the area has nice shinny copper ---- and wipe off with a wet rag.
Depends who you talk to .
Is it harmful to the pipe or the sweat joints?
There's plenty of speculation mainly on internet forums this type of situation is the source of leaks.
I can say from experience I've seen plenty of 50+ year old soldered copper with the green corrosion that hasn't shown signs of leaking from residual flux. Usually will find issues from poor soldering.
However --- if a leak is present this too will show signs of green corrosion.
Typically a leak will be somewhat easy to identify --- either by the area being wet and/or white crusty calcium buildup ---depending on the water quality.
In this case this calls for a repair to be done. That joint would need to be disassembled -- cleaned -- new flux applied -- re soldered -- followed by wiping off the excess residual flux after soldering.
Hopefully this helps.
"" an ounce of perception -- a pound of obscure "