This is a mated pair of red snapping shrimp, Alpheus armatus, hand collected from the Florida Keys. These are small, colorful shrimp most often found living with corkscrew anemones, Bartholomea annulata. The most distinctive thing about these shrimp is their snap, which sounds like the tank is cracking open (it also gets your attention when they snap their claw in your hand or on your probing finger, so be careful). Although the snap is strong enough to stun a small fish or shrimp, its mostly a defensive action and they are probably harmless to most of the normal inhabitants of a reef tank.
Each of the shrimp is about 1.25 long and both are close to being full grown. In your tank they will find a defensible corner or hole and pretty much stay there except fro feeding times. They may do some sand excavating, so if you have a lot of unsecured live rock, they may cause it to shift if they undermine some of the base pieces. They are easy to feed and will accept most prepared flake and pellet foods, and will scurry around the bottom cleaning up bits of stray food during and after feeding times.