Don't let a mantis creep you out (sounds like a good possiblity that you have one in there). They sell traps. When you catch him, depending what kind it is, you can throw him in your sump or fuge if you have one - or even a cheap 10 gallon tank. A local reefer might take him off your hands too. Some have tons of personality and are great little pets (again, depending what kind). I recently posted about a mantis-only tank that I just set-up with a 5" peacock mantis and it's probably my favorite fish (well, creature) that I've owned.
Either way, if it IS a mantis, you'll have to remove him from your system or your clean-up crew (and possibly more) are goners.
- Skins