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.
50 Gallon Oceanic: 2 maxijet 1200's, 2 150 watt visitherm heaters, BakPak II Skimmer with PS & BT, Coralife Lunar 2X65 watt Power Compacts
Filtration: 50lbs LR
.com, 60lbs sand
Swimmers: 2 O. Clowns, Yellow Watchman, Firefish, Yellow Tang, Dottyback.
Clean-Up: Several snail & hermits, Skunk Cleaner, 2 Peps.
Corals: 2 zoo colonies, yellow polyps, star polyps