Brown and hairy crab doesn't sound promising. You can tell what the crab is SUPPOSED to eat (capitalized because crabs seem to end up eating whatever the heck they feel like) by looking at it's claws. If they come to a sharp point, it's a meat eater and likely to cause you problems. However, if it has a round disc at the tips of his claws, then it eats algae (the pads help it grab the algae), and MAY be safe to leave in your tank. As to catching it, chances are the easiest way would be stalk it. Wait for it to come out in the open, probably during the night, maybe add some food to the tank at night to entice it to come out and then grab it.