Red clawed crabs need to occasionally dry out thier shells to control fungi.
They are also escape artists. They can get out of the aquarium and take walks.
Unfortunately they will not try to get back before they dry out and expire.
They can climb any tubing or filter inlet pipes. Thier "feet" are so sharp they
can actually climb the silicone sealer in the corners of an aquarium. I do not
know if they will get along well in a cichlid tank. Your missing crab was
either dismembered, the remains hidden in decorative rocks or something; or it
went for a walk and is under or behind some furniture in your house. Sometimes
the little escapees will go into the bathroom if it is damp there and hang out
until you pick them up. If it has not been more than three or four days you
might be able to retrieve it before it croaks. The crabs are good cleaning
crewmembers like shrimp or snails. And they will rearange the smaller
decorations on the bottom of the tank from time to time. They are entertaining
additions to certain tanks. Golden fidlers are another type of freshwater crab
that some people occasional have in thier aquariums.

Crabs, of course, spawn in the surf zone of the ocean, freshwater or not. So
they are not a candidate for breeding. As far as I'm aware no one has taken the
trouble to simulate the surf zone of a beach or the light conditions to see if
it is possible to fool them.