It could be a mantis shrimp, however it could also be a pistol shrimp. If it's a clicking noise you hear that gives it away, that if it isn't a pistol shrimp, then it is a basher type of mantis. I would try to ID it first to be sure there was a mantis in the tank.
Personally I would try to live catch it by rinsing out rocks to flush it out into a bucket and then sell it locally to someone who wants it. Often there are local
SW keepers that will pay for a mantis shrimp depending on the species. The common peacock mantis, the ones that can actually break a thinner glass tank, tend to be easier to sell. So to are the Gonodactylus Smithi species, which often don't get longer 3" in length but are very interactive and colorful, people often go for this species because it is smaller and does not pack the punch of the peacock and so can be kept in a glass aquarium.
I keep a Gonodactylus Platysoma in a 10 gallon aquarium and mine has actually hit the glass a number of times and I can't even feel it through the glass with the species I have.
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