I've never known a plumber to clean a joint after soldering so the "green" could very well be from the solder. You can clean that off easily with a mild acid solution. The most effective is muriatic acid which will leave the copper as clean and shiny as the day it was made BUT be cautious with muriatic - the fumes are bad and it will burn skin. Dilute it and wear rubber gloves. Dip a rag in it and wipe the corroded area. Rinse with another rag soaked in clean water. The results are instant - no soaking and waiting required.
I have been a plumber for 35 years, and yes I wipe every solder joint. We have never met but now you know att least one plumber who takes pride in his work.
To answer the OP"s question, brass is a neutral metal which will not corrode when in contact with steel threads on a water heater.
I would not mix CPV with the copper as a dielectric union.
Ed
