Welcome to R2R!
Calico Crabs definitely need a sand bed to be happy, they won't be out and about during the day much as well. Most of the time you will just see their eyes and antennae poking up out of the sand. Not reef safe as they are opportunistic feeders and corals can't run away. Snails will also be at risk; it's not uncommon to see a related species here in Pensacola called the shy-face crab munching on lettered olive snails and luna snails even during the day. They shouldn't bother bigger fish but smaller and weaker/slow-moving fish may be at risk.
Crabs are pretty much the garbage cans of the sea, so they will eat just about anything you give them. For your crab I would give it whole silversides, chopped squid, clams/mussels on the half shell, and perhaps a little bit of nori from time to time
They're definitely a cool looking critter! Good luck with the little fella, it should be fairly hardy!