how much rock? how old is your tank ? are you cycling?
If you bought it from a friend and it had a bunch of hitchikers why don't you just put it in your tank and let the crabs stroll out ,it sounds like it was mostly cured and that it came from a stablished tank, so it will have minimum die-off .But still watch ammonia levels for a few days and do a waterchange if needed. then aquascape and watch ammonia levels for a few days & finally do a waterchange if needed. If ammonia/nitrites are not there then you're good to go. Also keep Nitrates at a low level, close to none..