The green scale rust on the brass fittings is actually a protective mechanism. Commonly called tarnishing or patina, it is a result of oxidization. The brass contains too many negative ions, so when it is exposed to air it draws out positive ions from the air, beginning the process of oxidization. The result is the green scale found on the fittings.
Process: Why it Happens
The process begins when the metal is first cast. Brass is a combination of zinc and copper, which is why it has that light yellow color. Once forged, the metal is not balanced in molecular terms, so the molecules are forced to stabilize themselves. The easiest way to do this is to take positive ions from the air around the metal and form a more stable metal.
How to Stop It
Brass will always tarnish no matter where it is placed. Some people enjoy the look and refer to it as a patina, which adds character and charm. In plumbing fittings, tarnishing lends no charm or character. So, avoiding the process is a priority. Some factories apply a clear coat to the fittings before they are shipped. This seals the metal from air exposure reducing the chance for oxidization to occur. If the coat wears off, the process will occur. New fittings with a metal-based, micro-thin coating resist tarnishing for the life of the fitting. Or, the tarnish must be cleaned off.
Use a rust remover, paint stripper or citrus based product to remove tarnish from fittings. Vinegar also dissolves tarnish. Wipe the tarnish away with a clean rag and one of these products. Brass fittings that do not have a seal coating to protect them from the air will always require regular cleaning.