Interphasic creatures that clung invisibly to the Enterprise crew and fed on their cells, revealed only through Data's strange dreams.
These organisms lived in a state of interphase, out of phase with normal matter, so they could not be seen or detected by ordinary means. They attached themselves to crew members and fed on cellular peptides.
In Phantasms, Data's new dream program produced disturbing nightmares, including images of crew members being eaten. The visions were his subconscious mind noticing the creatures, and in one episode Data stabbed Deanna Troi. Geordi La Forge built an interphase pulse that cleared the organisms from the ship.