This is a nudibranch — what may or may not be a Pseudobiceros gloriosus, and there are many, many kinds of flatworms that look like this one. Check out DiscoverLife.org’s gallery of flatworms.
This National Geographic linkΒ also has some excellent nudibranch photos. And for a previous video study inΒ locomotion, check outΒ the Ribbon Eel.
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