That is a Sacoglossan Sea Slug, found in French Polynesia. It is one of the many kinds of life documented over a 24 hour period in one cubic foot underwater by photographer David Liittschwager. 

National Geographic has a great (flash) interactive photo gallery, including videos(!), of the five ecosystems that David explored and documented: forest in Central Park, a coral reef in French Polynesia, a Costa Rican tropical cloud forest, South African mountain fynbos (a collection of plants that are mainly shrubs), and a fresh water river in Tennessee.

via Raul.