Your larger crab, what does it look like. This may cause problems in the long run if he is not the right kinds crab. For example, eating your clownfish.
One the other note. Its does sound like it to me. There is no real way to treat him inside your tank because of your live sand and live rock. What you could do it raise the temp in your tank to about 85 to speed up the life cycle of the tank. But you have to do this slowly. you cant just crank up the themometer. You could also get a garlic additive and soak thier food in it. you should be able to find it at you lfs.
Ich is like a normal occurence in saltwater tanks. As long as you make sure your fish are eating, they need to keep up thier immune system to fight the ich.