The Cytoplasmic lifeform was an alien which USS Voyager found on a stranded vessel in 2375. When it was beamed to sickbay, it attached itself to B'Elanna Torres, piercing her neck and secreting fluids into her, using her vital functions to sustain itself.
In order to save Torres, The Doctor created a holographic image of a Cardassian doctor named Crell Moset, a real-life expert in exobiology.
Together, both holographic doctors determine that the creature is of high intelligence. Although Moset wishes to administer a neurostatic shock that will kill the creature, Voyager's Doctor administers a shock that immobilizes the creature without harming it. Once detached from B'Elanna, it is returned to a ship that was sent to rescue it. (VOY: "Nothing Human")