Brightly colored flatworm filmed in the waters of Sulawesi, Indonesia

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