It's going to be pretty hard to pinpoint exactly what kind of crab you have there, as there are over 4,000 described species of true crabs. As far as there being more? Sure, it's a possibility - but most likely that one hitched in on a piece of live rock. Just keep an eye out...it's the best you can do at this point. You could try putting a piece of shrimp in your tank after lights out and watch to see what shows up - that is a good way to draw most crabs out of hiding.

W.