thats probably not your problem crabs are scavengers and will eat a varaity of things. left over food, algae etc....
if they are dying i would suspect either they are killing each other, water parameters are not good or you have another predator in there that you may or may not even know about
whats your fish and invert list and water conditions like?
if it is starvation you can try spot feeding them or maybe placing an algae sheet on a rock for them to get at.
I dont see mine eat algae to often and mine would starve then cause i have very little of it at all.