This is a Cuthona yamasui, which is a species of sea slug, or nudibranch. They are a shell-less marine Gastropod mollusc. I found this colourful little guy (about 3 cm long) while doing a night dive in the Komodo National Park. It appeared to be eating the white Stinging Hydroid (Aglaophenia) it was sitting on.
Komodo National Park, Indonesia
Nikon D300, Nikon 60mm lens, Aquatica underwater housing, 2 Sea & Sea YS110 strobes, 1/100, f20, ISO 400, spot metering, manual exposureby Gary James Brennand, Western Australia