Hard to say but could be a prolapse. Try and soak any foods in a little Epsom salt solution (1/2cup distilled water and 3/4 tsp Epsom
Salt).
+1 Nope your completely on point. Water quality is an obvious one but there is so much misinformation on flowerhorn foods. Majority of people promote high protein, beefheart, etc. Like any other cichlid, all these fish need is a high quality medium protein pellet, (40% crude protein compared to the 65% I am seeing on some of the flowerhorn pellets being promoted) an adequate sized tank and filtration, and 78-82 degree water.It can due to high protein diet and water qualities in my opinion. I treated the tank with epsom salt, wc every few days and went backed in again. It happen when i tried a high protein diet pellets. Some experts here can chime in.