Mantis are not maddog killers, they are just good hunters. Would you keep a tiger in the same barn as your flock of sheep?
But to your question, if you do not want the mantis to be the star of your tank, then you need to remove it. There are people all over looking for mantis, so depending on coloration, please do not killit if you do not have to. Someone will want it.
Make a shrimp catching trap...keeping in mind that it will catch any shrimp so keep an eye on it and remove teh ones you do not want caught.
Do you know where it lives? Is it in a hole within some live rock? If this is the case then you can get it out fairly easy. Take the piece out of the tank, hold it over a basin (taking in account that it may jump off so make it a wide mouth basin), and introduce a stream of RO or RO/DI water into the hole. The mantis should come flying out fairly quickly.
DO NOT PICK IT UP. They are called thumb splitters for a reason. Imagine trying to grab a moving scalpel by grabbing the tip, don't do it. Just scoot it into a glass or container.
If you have a small tank that can host a species only animal like the mantis, keep it. They are a blast to watch and will provide hours of viewing pleasure, googly eyes and all.
If you can not work out the rock/water thing, then try a shrimp trap. A clear cleaned (without chemicals using RO or RO/DI water) soda container. At the point where the "bell" shape forms into the body of the bottle (allowing for a slight curve) make a straight cut around this bell and reverse the bottle top. Now there are several ways to secure the top. The way I use is a hole punch in 5-7 spots, and I lace in fishing line leaving a 2-3' piece to act a s a tag line hanging out of the tank tied off for reeling in the prize. Take this trap put a piece of shrimp or other bait and fill with tank water, sink near the suspected mantis hole and wait. It can get in but can't get out.
If caught this way, you remove and then unlace the fishing line and place in shipping container or small tank (a little 10-15gl with a simple $10-15 dollar biowheel/waterfall filter or the like would be fine for most of teh liverock hitchhikers).
Good luck.
Ray Boemler
www.peanutbutterjellyfish.com