The chances of there being a mantis in that tank are slim to none. I've looked that rock over more than most parents look their own kids over when they are born. It sat on my desk for quite some time and I saw nothing.
It's a 10 gallon holding tank, and has 1 hunk of rock. Nothing else in it but some corals, crabs, snails, worms of some type (pinkish), and pods.