Does galvanic corrosion on a hot water copper pipe travel from point of contact, 2' 10' 20' in solid or spotty patterns in both dirrections. Is it more pronounced elbows and fittings. Would a galvanized chain attached to a copper hanger (chain not touching the pipe) or galv chain supporting the pipe separated by rubber so there is no contact be a problem. I don't have a picture but at first look the greenish pattern looks as if the plumber had dirty fingers (flux) and years later these fingerprints turned green running down the pipe but covering the fittings and more so near the ends

Rubber separating the galv from the copper would be just fine. The green you see is most likely just what you suspect, leftover flux. If its not leaking, I wouldn't worry too much about it. It certainly doesn't look good, but usually is harmless. If you are worried about the pipes being weak, shut the water off, and give the pipe a good poke with your pocket knife. If the pipe dents easily, or you can poke your knife through it easily, you have a problem. If it seems nice and rigid, you should be okay for years to come.

Greenish copper is no big deal. Grab a flashlight and look at the printing on the sied of the pipe, what type is it. Type is L, M or K

The Dirty Fingers theory may be it. The greenish "stuff" on and around copper fittings indicates that plumber didn't wipe off flux after soldering. Take steel wool and clean it. Milo.