Was the other Crabs leg broken off, did you see it missing? Sometimes they are eating the molted /discarded exoskeleton, and it looks like the body part.
Are the crabs in brackish water, do they have a space to get out of the water as they are partially terrestrial?
Crabs are scavengers and will eat whatever they can. They will eat many kinds of food, meats and veggies, prepared foods.
Care must be taken as to keep the water in safe parameters, especially if you are feeding things in a trial and error manner. You can remove uneaten food after a couple hours - a turkey baster is really useful for that.
There are turtle type in tank filters, is there a filter on the tank and are you familiar with the nitrification process?
There is a "Getting Started" link in my signature which is still similar to keeping fish.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves