I too would love to know the true answer to this question as I have searched and searched for it. My largest turned white a couple months ago, yet the smaller two have kept their green color. I'm thinking it might have to do with age.
I had an emerald crab turn white on me also. Two weeks later it died. I attributed it to a lack of nutrition (bubble algae, etc) for the reason it turned white, but i really don't know. I had 3 total that did this.
Could it be because of his diet? I have read that they are green mainly because of the type of algea that they eat.........I may be COMPLETELY wrong on that one......just my .02