Magnolia,
I'll give you a small heads up. I hope you find the crab and get him out, they are bad news. During my first years in reefing I remember getting excited when I learned I had a crab with my live rocks.
I didn't know that hitch-hickers in the reef world are bad news. Well this crab eluded me for months. I kept losing my snails and then my fish one by one in the night.
Finally I stood up one night and saw the little hairy bastard. I couldn't get him because he was so embedded deep in my 80 gallon tall reef tank. So out of frustrations I went to my 10 gallon Octopus tank and asked him for a small favor.
Yeah I dropped him in and bam in just 10 minutes he caught the litter bugger and had sushi right in frong of me. I would have joined him but I just had finished a cheeseburger, fries,,, hehee
Now I know you won't do that but you should find a way to capture him before it becomes a problem...
good luck,,,
Mike