Well, okay then. Here are some things to consider.
If you have no other Cichlids in this aquarium, the smaller Severum may actually be bullying/harassing the bigger, green one. I've noted a trend lately that red colored Severum seem to be a bit more aggressive in temperament than the 'normal' green colored ones are. Unfortunately, this aggression may not be all that evident during the day with lights on, as these things can typically happen at night when lights are off. Your red Severum may be establishing dominance over the green one now. But, if your green Severum is eating and swimming normally (when it wants to), and shows no physical signs of damage.... then it might be best to observe things closely and hope this works out long-term for the best.
Food. If you are not already doing it.... Severum will appreciate a more veggie-based pellet and flake diet. Purchase stuff with a high content of Spirulina in it, that would work good for feeding African Mbuna. Keep your food fresh by refrigerating it, and if it is more than 6 months old replace with a fresh supply. High quality food sources will help to boost up a possibly stressed fish' immune system, keeping it from getting sick or overcome by parasitic attack it would normally be able to resist.