Are you talking about the Naso tang? I see a yellow and a Naso tang, I think the Naso tang looks like it has an injury and perhaps an infection but I need better photos to know for sure.
If it’s an infection, it’ll need to go to a quarantine dosed with furan 2, kanaplex, and metroplex. First, an acriflavine dip would be helpful.
Don’t use melafix for anything, it’ll just mess up the water and stress the fish, IME