From Creatures Wikia
Nudibranch (Rockinium silveria)
A more unusual member of the Marine Terrarium, the graceful, plant-like Nudibranch was referred to as the 'Twisting Dancer of the Seas' by some shee. It floats around the underwater cavern looking for food, and is completely harmless to creatures.
At the beginning of the game, several nudibranches are present in the left side of the Marine Terrarium. They do not lay eggs, but rather die (with a humming noise which is startling if unexpected) and respawn every now and then.