Very weird and cool looking thing. I have no clue what it is, never seen anything like that before.
If you can't figure out what it is and nobody here can, try sending an email to the pros at WetWebMedia.com. They can usually figure out a lot of the ID's.
Maybe a Male Sphaeromatids Isopod??? Check out: Pills, Parasites, and Predators; Isopods in the Reef Aquarium by Ronald L. Shimek, Ph.D. - Reefkeeping.com