Looks like a Dermatobranchus sp., but I'm not entirely sure which one. My best guess would be Dermatobranchus diagonalis, but - regardless of species - if it is a Dermatobranchus sp., it's not reef safe (they eat octocorals - softies, gorgonians, GSP, etc.).
Some other (at least sometimes) relatively similar species:
Dermatobranchus striatus
Dermatobranchus caesitius
Dermatobranchus dendronephthyphagus
Dermatobranchus earlei
Dermatobranchus funiculus
Dermatobranchus phyllodes