Poor guy, he looks terrible! Even the other fish seem stressed out by his illness. I'm sorry I have no idea what the problem could be, maybe a bacterial infection? Maybe posting this information will help someone else diagnose your fish.
Tank size:
pH:
ammonia:
nitrite:
nitrate:
kH:
gH:
tank temp:
Fish Symptoms (include full description including lesion, color, location, fish behavior):
Volume and Frequency of water changes:
Chemical Additives or Media in your tank:
Tank inhabitants:
Recent additions to your tank (living or decoration):
Exposure to chemicals:
Digital photo (include if possible): (You've already provided a video so probably no point posting a photo).