In addition to J.A. Streich's correct answer, saying that the ghost can't use spell slots, I'd like to make a few more notes.
The ability says that
It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.
I would argue that using a spell from a racial ability is using the target's "knowledge", and that therefore the ghost shouldn't be able to cast those either. Another DM might argue that racial abilities are inborn, and that the ghost should be able to use those, the same as it is able to walk while possessing a creature with legs or fly while possessing a creature with wings.
Initiative and Turns
The ability also says that
the target is incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness.
So, the target still gets its turn, but it is incapacitated and therefore cannot take any actions or reactions.
The ghost would then control the character on the ghost's turn, which is still on the same unchanged initiative. This further implies that the ghost can't take any actions using the character's body on the same turn as it uses an action to possess the body. It must wait a whole round, giving the possessed character's allies each a turn to try and do something about it.
It might still be able to move after possessing the body if it hasn't already moved on the turn, however, which could be bad if there is some dangerous terrain in the area.