The classification of this nudibranch is formally known as Nembrotha kubaryana
. These stunning creatures and their snail-related cousins form one of the main divisions within the Mollusca. Most are characterized by the combination of a pair of tentacles on top of the head and a tuft of feathery gills on the rear part of the back.
One would think that these soft-skined, unprotected creatures would have difficulty fending off predators. As usual, nature invented a solution. The flesh of nudibranchs is often toxic or distasteful and the bright colors seem to serve as a warning sign. Some forms of nudibranchs feed on various hydroids and other cnidarians. They're able to digest the stinging cells (nematocysts) of their prey without firing them and incorporate the cells into their own tissue. Thus, the feathery protrusions covering their back are armed with stolen weapons.
Source: Allen, Dr. Gerald R., and Roger Steene. Indo-Pacific Coral Reef Field Guide. Singapore: Tropical Reef Research, 2002.