How long has your system been established for? Emerald (Mithrax) crabs are very dependent on their main source of diet, which is algae. Hence the coloration of their their bodies due to the supplementation of this food source. When the necessary sources are depleted or not provided, the crab looses the green coloration, pretty simple. Thats not to say that because there is no algae for it to eat, it is deprived and starving. But almost all will turn white if this continues on.
Easy fix. Your LFS probably sells Macro-algae sheets of pressed and dried algae that is garlic infused (to fend off Ich.) They come in Red and Green packages with many sheets inside. The Red pack is better suited for Angel's, Wrasse's, Goby's and other fish, the Green is more towards Tangs and inverts. Get the green pack and tear a sheet in 1/4's, then place it under something heavy like a piece of LR. In no time your little guy will make his way over there and feast, over time, the coloration will start to return.
Let us know how it goes!

Happy reefing!