The lettuce sea slug(Elysia crispata) is a large (5cm) and colorful species of seaslug, a marine gastropod mollusk. The lettuce slug resembles a nudibranch, but it is not closely related to that clade of gastropods; it is classified as a sacoglossan, or sap-sucking sea slug.
Sacoglossans live by ingesting the cellular contents of algae, hence the adjective “sap-sucking.” (xxxxxxx)