I've been researching the nutritional value alfalfa Medicago sativa (aka lucerne, lucerne grass)which is considerably high, often being called a "superfood." It sprouts easily and can be sown directly into substrate in a crab tank (if substrate is not wet with saltwater) for a live plant feature. But the deep root system of mature plants yield a higher mineral content than sprouts. All parts of this plant are safe an nutritive.
NOTE: Sprouts commonly found in grocery stores sold for salads might be alfalfa, clover, or a mix of radish and broccoli. All of these are safe for crabs.