This little guy is Elysia chlorotica, a species of sea slug that is able to usurp the chlorophyl-producing capabilities of the algae it eats. For this reason, they are nicknamed "solar-powered sea slugs".
Through a process called Kleptoplasty, the slugs are able to hang onto plants' chloroplasts (the cell organelle responsible for creation and storage of chlorophyl), keeping them intact in specialized cells in the slugs' digestive system. Amazingly, the slugs are also able to incorporate the chlorophyl-producing genes from the algae into their own genetic makeup. This allows them to make use of the burgled chloroplasts. The exact mechanism by which this is achieved is still unclear, but by the end of it, the slug can live off of light alone- up to ten months on the same alga's chloroplasts!
E. chlorotica is the only example in the Animal kingdom known to be able to do this. Incredible, the solutions evolution can come up with to overcome problems in an organism's environment! I, for one, am going to be eating more algae.