Tough to see, can you take a picture under white light?
How long have you had the fish?
How long has it had the bump? Did it start suddenly, or grow over time?
Any chance that it ran into something in the tank?
It looks to be an injury with a subsequent infection, could be bacterial, but there is an internal fungal disease known as Icthyophonus, that can erupt up through a fish's scales like that. It isn't treatable though.