Nasty crabs!!! HELP!!

[tplouffe]

I recently received some used, live rock which has housed 2 crabs. They hide all day (obviously), and just happened to see a white one moving from the "cave" in the new live rock.
Google says to use Krill tied to a small rock placed in the bottom of a cup. Going to give that a try, I just started my tank 1 month ago, so don't want to lose the fish I have.
Any other ideas would be greatly appreciated.
Thanks!