For me, the only crab I would let live in my tank is an Acropora Crab. Other okay/good crabs include: emerald crabs, anemone crabs, pom pom crabs, hermit crabs which aren't true crabs.
Any other unidentified crabs are potential coral eaters/coral destroyers. Good tip for hitch hikers. They either live with or live on (eat) whatever they came in
Look at Jruga's crab which moved all of his frags, ate his zoas, and his green star polyp. Not something you want in your tank. We also had another member here who bought a frogspawn colony and discovered a stone crab living inside the skeleton of the coral.
Anything else gets put into the freezer for relatively humane death and then the trash can.
