All crab are omnivores. Some crabs are more prone to eat meaty items and others algae.
Emerald crabs prefer algae. But, if your tank doesn't have any algae, then they will take the opportunity to eat other things instead of starving. That said, all crabs will have preferences on the type of algae they prefer as well. So, they may not devour very type of algae in your tank.
If in doubt, best to feed your crabs on occasion.