This is a dorid nudibranch (meaning naked-gilled), Phyllidia sp. The halos surronding the spiky protrubernces appear to be normal coloration. Nudibranchs are basically shell-less snails and are specialized feeders on coral polyps, sponges, and other reef inhabitants. Many can transform their food into toxic substances. They can be disguised and hard to see, but this one and many others are very conspicuous, perhaps as a warning to predators. Dive site Southeast Pantar, Biangabang, Indonesia.