Welcome to AA, Most crabs are unwanted hitchhikers to a reef tank and as they grow can become a real danger to your fish and inverts. They should be removed and if you have a sump you can put them there. Here is a Quote on how to get rid of them:
Originally Posted by
Scubaman 
there are easier ways to catch a crab - just find a glass and stand it up against the rockwork, then put something tasty in the bottom. Wait overnight and you should find a crab in the bottom and since the sides are smooth and they can't swim, it won't be able to get out. Way less of a headache than the methods you're all describing, plus the crab will be unharmed and can be put to good use in the sump or fuge.