This is a nudibranch or sea slug, sometimes called a Spanish Dancer, — what may or may not be a Pseudobiceros gloriosus. Either way, it is a flatworm! And there are many, many kinds of flatworms that look like this one. Check out DiscoverLife.org’s gallery of flatworms.

This National Geographic link also has some excellent nudibranch photos. And for a previous video study in locomotion, check out the Ribbon Eel.

(Update: Now this is a Spanish Dancer!) Thanks for the correction and the video, @VaranusSalvator.