A sea slug
Elysia chlorotica is one of a species of sap-sucking sea slugs, Sacoglossa, which feed on algae but extend beyond just feeding on the fluids and absorb within their own tissues the algae's living chloroplasts and their DNA. Elysia chlorotica in particular is bright green, looks like a leaf, feeds on sunlight like a leaf (by virtue of the chloroplasts) but is in fact an animal.
