A shrimp or lobster. Can't tell you if it is reef safe. A shrimp iwould say yes a lobster i would say it would eat all your snails and crabs and shrimp. Maybe put it in your sump or QT untill you can find out what it is.
Looks like it may be a Callianassa sp. ( Ghost Shrimp) or Axiopsis cf. but at that stage it's hard to tell. If it is a ghost shrimp it is reef safe they are good for aerating the sand bed. Could also be a pistol shrimp.