WTF. He's sick. Or its a bite wound. Is there a bigger fish in the tank? Or Some kind of bacteria infection. Lower the temp slowly below 80 because bacteria grows faster at high temperatures
There is a shortbody texas cichlid in there with him. i separated them and now i have the flowerhorn in a quarantine area. hopefully this wound/infection goes away
Do methylene blue baths twice a day, and also add it to the tank water. Do daily water changes with 1 tbs salt per 10 gallons (with the mb). If you have furan 2 you can add that too as it is a mild antibiotic and can be used with mb. If things go down hill use kanamyacin as directed.