Well, would you look at that. An actual mantis shrimp hitchhiker? Looks like you have some aiptasia on the rock as well.
Since I'd assume that this guy's life isn't what you intended for your system I'll assume you want it out. You can get a tall glass and put some food in it to try to lure it out or you can take the rock out and simply submerge it in freshwater. The freshwater dip will make sure anything living inside the rock makes its way out. Not the most humane way, but it will solve having an unwanted predator in your tank.
EDIT: You could take the rock out and hit it with a hammer to get it out too. Could keep it in a sump or try to trade it into your LFS for credit.