It's impossible for me to say, but with that strange mix I imagine it could be a fairly high stress tank, and it will probably give you nothing but problems. Resting imobile in a corner is a sure sign of a fish trying to keep out of sight, which in turn is a sure sign of stress.
I don't know if you know, but the Africans have very different diet then the new worlds, and they're behavior patterns are equally incompatible.
It could be a problem like pop-eye, or it could be that it got damaged in a fight, it's impossible to say from over here. Either way it would be a good idea to seperate it and treat it. If it's only damaged then simply keep the water clean and it should heal - if the eyesight is gone it will probably be permanent. If it's a bacterial disease like pop-eye there is medication available, but make sure you treat in a seperate tank or you'll kill the biological filter - plus there's no reason to expose the other fish to the medication.
In the mean time, seriously consider reassessing your tank to avoid future problems. As the fish grow and mature the stress levels will be very high. A 75 gallon is a minimum size for a pair of Texas, OR it's a nice size for a pair of JD's, OR for two pairs of smaller cichlids like the firemouths. People are amused by the constant fighting, but fighting is stress, and stress leads to disease. Besides, pairs are the most rewarding way to keep these new worlds, the moody interactive behavior between couples is what seperates them from most freshwater fish.
If you decide the fish store arena style aquarium is more for you, at least remove the Texas because they're too big and aggressive for that setup in the long run, and remove the Africans for reasons already mentioned.
Texas aren't more prone to disease, unless you got a weak strain, but it could be that he's taking more abuse... or it could simply be coincidence.
