Hi and welcome to the forum
Have you checked the water quality for ammonia, nitrite, nitrate & pH?
If yes, what were the results (in numbers)?
Can you post another picture showing the fish from the side and not against a blue backing?
It could be colouration, excess mucous or something else. But if the water is good and the fish is still eating and acting normally I would just monitor over the next few weeks and if things get worse, post another picture.