imo it will not harm the crabs but keep a close eye on the water quality and do weekly 25% water changes with RODI water.also try to remove as much as you can when syphoning.
once your nitrates are down to 0 for a while the red slime should "starve" and go away, another good why to control slime is to increase water flow in the tank. slime likes slow moving water and the turbulance will keep it from growing on the rock. ...........good luck......al