Nudibranch of genus Elysia feed primilary by piercing and sucking algae cells (chloroplasts).They are sap Sucking Slugs.Thats why Elysia is,like their food, often green.Young species are brownish or beige in colour.
Elysia obtusa grows to about 2,2 cm long. The body colour is a translucent yellow,yellow-green or green covered with white spots. The broad parapodia are distinctively rimmed with white. They are sometimes a much darker shade of green than the rest of the body (pigment of the chlorophyll shows through).