This is Costasiella kuroshimae -- the Leaf Sheep. It's appropriately named because it looks like a sheep with green wool. What makes this little fellow so amazing is that it can eat algae and use the chloroplasts inside to absorb energy from light. Ameena Schelling writes for Discovery:
Leaf sheep retain the chloroplasts from the food they eat and use them to manufacture their own energy — just like a plant would. The process, known as kleptoplasty, is only found in certain sacoglossan sea slugs. While leaf sheep aren't particularly good at photosynthesizing, some species can live for months on photosynthesis alone.