Hypo refers to the term “Hypomelanistic” which means “a less than normal amount of melanin” and describes a condition of reduced pigmentation. The mutation manifests itself as a lighter colored, “ghostly” looking animal - hence the original slang “ghost”.
Historically there have been Hypos, Orange Ghosts, Blue Ghost, Green Ghost, and so on. They are all technically “hypomelanistic”. Some of these lines (such as the generic hypo and orange ghost) have proven compatible but were derived from different lineages. In this case you could compare it to the difference between coral glow/banana. In other cases the lines are not compatible and represent different genes or mutations that only resemble each other visually. Hope this helps!