Not contagious because it is caused from a poor diet. Malawis diet should be based primarily on veggie matter. Spirulina is a great flake for malawis. Frozen and live food should be fed only sparingly because they cannot digest all of the fatty foods and proteins that food with fish contains.
Mbunas, IME, always eat like pigs if they are healthy, and it would be very easy to overfeed them because they always appear to be hungry. Overfeeding is probably the #1 killer of all fish, so if the fish looks fat then he is eating too much. I would reduce the feedings for a couple of weeks and see if he slims down some.