The intellect devourer's Body Thief action states that the Intellect devourer "takes control of the target's body." We know that the victim's body is still alive and counts as a creature.
The ghost's Possession action states "The ghost now controls the body but doesn't deprive the target of awareness."
So does the ghost override the intellect devourer to control the humanoid in question?
Also, in terms of the Charisma save, would the save automatically fail since the target is ultimately the humanoid, or would the intellect devourer attempt the save with its own Charisma to try and maintain control of the body?