Hermit Crab discolored and continually leaving shell

Question
One of my hermit crabs has begun regularly leaving its shell.  Initially, I thought it was dead because it was laid out in the bottom of the tank, but it wasn't.  It is discolored (whitish) on the back half of its body and is pretty lethargic.  Periodically it will put on another shell, but doesn't stay in it.  It also doesn't seem to have the strength to move around much.  

It is drinking water and it stays by the food bowl, so I assume that it is eating.

I looked for an antifungul, specific to hermit crabs but couldn't find anything at my local store.  Thank you for your suggestions,

Answer
Their abdomens are whitish because they are moist all the time. Are your humidity and temperature between 74 and 80? There are no antifungals that are safe for crabs. A dip in some ocean style salt water is your best bet but also making sure you're environmental settings are correct and stable.