It’s a good thing you spotted the mantis shrimp now — a one-inch stomotopod is much more manageable and much less of a threat to your seahorses that a large specimen.
Mantis shrimp can be trapped but you have to be patient and persistent to succeed. They are cautious animals and often need time to become accustomed to the presence of the trap, so you might have to be your trap for several days in succession before you bag the little bugger. A number of commercially made mantis traps are available, but you can easily fabricate your own from items around our house. For example, the following link shows how to construct a simple trap for stomotopods:
There are quite a number of other methods that often work well for killing or capturing a mantis shrimp if you happen to know where it holes up. See the following website for a good discussion of various other techniques for removing mantis shrimp from your aquarium:
Click here: OZ REEF – Mantis Shrimp
One of the methods described in the article above should work well for you, Carrie. Best of luck bagging the little beasty!