Yep, that's definetly a mantis shrimp.. that one in particular looks to be a 'spearer' but a clearer pic will be needed to say for sure. They are INCREDIBLE creatures but are not good additions to most reef tanks. If it is a spearing mantis, your medium/ smaller sized fish will be at risk. If it's a smashing mantis (you would've probably heard a bunch of ticking noises by now) your inverts and bottom dwelling fish will be at risk.
Yup, Its a Mantis. We have one that we keep in a little 3 gallon plastic tank by itself. They are awesome little killing machines with really complex vision. Ours is a "smasher" and there are spots on the tank that are destroyed.
Cheers! Mark
You can try the glass jar tilted at an angle with the lip next to its rock, and food/bait at the bottom. I've tried putting Saran wrap over most of the mouth of a jar to keep fish out. You can also check out commercial plastic mantis traps (I think that someone makes one, but I'm not sure). Regardless, good luck - they are quite intelligent! You could also literally feed it like a pet, and over time as it gets to trust you, lead it to a glass jar trap.