Mantis shrimp

I recently started a new tank and bought some Florida aquacultured live rock ... I have had it for like 2 months now and I am pretty sure I have at least one or more matos shrimp in there because I keep hearing a lot of clicking popping noises. I haven't had eyes on him yet. Today I found a small crab in there in pieces.i tried the bottle trick to no avail any other suggestions? Thanks