I think there are two different morphs called "green ghosts".

You used to hear about green ghosts that where compatible with the larger group. I even seem to remember NERD posting once that he started one of the other lines (orange?, yellow?) by selectively breeding green ghosts. But recently I've heard of green ghosts that might not be compatible with most of the ghosts. The other incompatible line I remember is Graziani's G1. Oh, and reportedly the original name sake of the ghost mutation was something different but that line might have died out completely and been replaced by what we now call ghost (or hypo if you are a purest).