That is grape caulerpa. The things I know that will eat it:
1. scribbled rabbitfish [doubtful around lps] max size 10".
2. tripneustes gracillis, the hairy colored pincushion urchin: not your common pincushion, nor in the same family. Coral-safe. Putty your rocks, but a great algae eater, of all types.
3. some tangs. Be sure which.