The most common answer is to immerse the rock in fresh water and the bug will come out. I've tried this more than once and it hasn't worked for me. Plus, fresh water ain't good for live rock and I assume you still want the 20 pounder. how bout this? If the mantis is small and you know where he's hiding, take a pick and a hammer and chip out his cave area. Its probably not very deep. You would risk killing the mantis or splitting the rock though...your call
Option 2 is leave him in your tank and feed him twice a week. If he turns out to be nuisance then get rid of him. Mantis usually don't turn really aggressive until they get really hungry. Good luck.
