It can be a pistol shrimp. A buddy of mine had a mantis shrimp, and we were able to catch it with the coke bottle trap. Basically take a 1 liter coke bottle, cut off the top and flip it over so it creates a funnel inside the bottle. place a raw shrimp in the bottle, and secure the lid. Lean the bottle up against the rock you think hes in. It may take a couple nights, any you will probably catch a bunch of hermits, but in time, you will catch him. You can try multiple traps, me and my buddy caught his cleaner shrimp several times, thats why i would suggest multiple traps. Good luck.