Not sure what it is but if u see it is as a white type of growth then may more than a cut. Don't look like wooly fungus. Add some MB to the tank and keep an eye out. Maybe need a real close up shot.
What else if in the tank? If its solo I'd agree with treatment. If there's another payara of similar size I'd think it was a cut. My tats cut each other up and that's a common place.
In all honesty if an army is on top its usually eating the rest. They are very easy to bully because they have specialized teeth that aren't very good for fighting. Great for eating but not for fighting like a mala, gatf, or gar. If there isn't a mark on the other side of the fish then I'd take gar off the list as a suspect.