Emerald crabs eat algae in which is where they get their green color. If there is not enough algae, they tend to scrape off coraline algae to eat as well. If you do not have any macroalgae in your tank for them to eat, they will loose their color and eventually starve and die. If this is the case, simply add a bit of green algae sheet such as NORI or something for them to eat and stay healthy.