First you must observe your tank for a while to determine if U really have a mantis shrimp in there. It seems you are unsure. It might just be that your tank is cracking
( just joking ). There are other, less bothersome shrimp call " pistol shrimp" which make essentially the same spund as the mantis shrimp . Pistol shrimp can be a desirable addition to the reef tank because of their burrowing and excavation habits which keep the bottom churned up. They are also good scavengers. Their only drawback is the occasional snack on the small fish, particularluy in a small tank. In large tank they seldom pose any threat.
If your mantis shrimp will not show himself in your presence, you will have to lure him out. You should stop feeding the tank for at least a week, then put the bait (brime shrimp or clam meat) in a wide mouthed net place on front of the hole into which a mantis shrimp has retreated. When the shrimp enters the net, you've got him. Remenber, if you snag the net on the rocks in the excitement of catching him, and he ESCAPES, your shrimp will be so much harder to catch the next time.
Have Fun.