My wife put a crab in my tank - what is it?

[anchorlightsandpancakes]

After acclimation it mostly buried itself in the sand immediately. It hasn't moved for 24 hours except to grab sift through some sand in front of it. It's about 4 inches wide from elbow to elbow. Its pincers have been folded most of the time. Maybe 7.5 inches wide if extended?

Anyway, I have no idea what it is or if its safe for a tank. It's currently not in spare tank not my reef.