You need to wash everything thoroughly in the tank and change the sand or whatever. Rinse the crabs out well in very tepid water. They are dust mites in all probability. Not only do you need to clean everything out and replace the substrate you also need to find out why you are getting them in the first place. It needs to be replaced daily.
Spoiled food can create that.
Also, do not mist inside of your tank. Another harmful thing. Whenever possible you need to find the root of the problem to prevent it from happening again. Let me know if you have anymore questions or if I can help. You need to wash everything thoroughly in the tank and change the sand or whatever.