There are mantis traps like the x-terminator trap, I have heard these work OK. Otherwise keep an eye on your tank and when you see him peeking out from a rock repeatedly that may well be his chosen home for the time being, then you can dip that rock in freshwater or hyposaline water or fizzy water with the same salinity and he should shoot out.
I keep a few mantises both in their "own" tank and also the main tank, they are very intelligent and fast learning creatures, and as long as they have crabs to harass, they leave everything else alone. I hand feed mine little pieces of shrimp and they are actually pretty comical little guys because their eyes follow you around when you come up to the tank.
However bot h smasher and spearer type mantises will not hesitate too much to kill whatever they are hungry for including fish if the need arises. So they will need to be fed or evicted one or the other.