it's a thalassinidea missing one of the major chelae. These are the mud shrimp & ghost shrimp. They're more likely to be found at bait shops rather than LFS. Mud & sand are the usual habitats and as the burrows may extend for many feet & have many side chambers they are not at all suited for the usual small tanks. That's kind of a pity as they are quite reefsafe (detritivores, scavengers, and herbivores) and have interesting behaviors. Some are farmers - they collect drift seagrass & algae & let it decompose in special chambers of the burrow, then eat the microbial fauna that grows on it.