Hard to see clearly, need higher rez pix, but it 'looks' like it either jumped/smashed into something and injured itself, or possibly burned itself by hitting a heater (if you have one). Again clearer pix would help.
If it is a physical injury it should go away after time, but you'll have to try to prevent secondary infection by keeping water clean and adding a bit of aquarium salt. For burns I don't know what's the best treatment (maybe same), I haven't used a heater in well over 20 years. Hope he gets better.