It's a Phyllidia species, probably Phyllidia coelestis. There are several less common species that are similar, though. You can find some warnings on the Sea Slug Forum about keeping phyllidiids in aquaria, you might want to read up on them.
I believe it's a specialized sponge feeder, and will probably eventually starve to death. Read this about P. varicosa, which is a similar nudibranch, and may actually be the one you have:
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