I have multiple little crabs that hitchhiked their way into my tank on the live rock. I can't catch them or even get good looks at the to ID them but I am scared they will hurt corals that I would like to start adding. Whenever I attempt to get them they just hide deep in the crevices of the rock. Does anyone have any tips on how I can force them out? Thanks.

make traps like these but on a smaller scale

http://www.youtube.com/watch?v=6P_hI7drGSQ

http://www.youtube.com/watch?v=2gKizvpGKPk

Is a freshwater dip an option that would work? Does dipping rock in freshwater kill all the beneficial bacteria that make the rock "live"?

yes, if you dip it for very long.

Meaning it only works for the crabs if you dip it very long, or it will kill the beneficial bacteria if i dip it very long?

try the traps

+1 to traps. Inverted bottle traps work well. You should try to disguise them though. One guy on here used duct tape to connect the 2 sides and just stuck it in. Try to wedge it between some rocks so that it looks like a cave or something.

Be sure to get an ID on the crabs before destroying them though. Typically they are of use to someone. You could even freeze them, then mash them up and use them as food for your carnivorous fish/inverts.

I used the bottle trap method and caught a crab after a day. I pushed the bottle into the sandbed and threw some shrimp pellets in there. Works like a charm, just be patient.