Cassiopeian Dream Eater is an alien organism that attaches itself to the face of its host, then feeds off of the chemicals produced by the brain during stress by trapping the host in a nightmare.
The Dream Eater has a very strong grip on its host, as mentioned by Kevin in Night of the Living Nightmare it would be impossible to remove it without ripping the victim's face off. Presumably it can be removed by a Galvan doctor.