that is the single craziest HH i have ever seen. If noting was obviously being eaten I would have kept it for sure. They'll probably be the next borb. . .
Large predatory flatworm/polyclad flatworm.
http://www.extreme-aquatics.com/707/saltwater/how-to-indentify-worms-in-your-reef-aquarium/
Everyone should have a small pico refuge tank for all the fun stuff. Those gorilla crabs and Eunice worms are bad in the display, but really cool to watch in their own little place. People do it for mantis shrimp, why not the rest?