This flamboyantly-colored sea slug belongs to the family collectively known as "sea hares" (Aplysiidae). They are herbivores, feeding on algaes and seaweeds.
This individual was only ~25mm in length, not including the long (longi-) tail (-cauda) for which it is named. The species is adapted to living on floating seaweed. Sea hares of this genus can be very numerous, in the thousands, when conditions are right.
This lone individual was found in Piti Channel, Guam. In over 60 dives at this site, we have seen it only this once.