I got a polleni too with a similar looking damage. It has had it for a few months. The fish became markedly less interested in swimming around and feeding but still does both a bit.
Long term salt bath of up to 0.3% didn't do it. Neither has MicrobeLift Broad Spectrum Disease Treatment based on Malachite Green and formalin. Both treatments were done in situ, to all my tanks and sumps and the whole system.
I kind of gave up for the time being to see what will develop.
I did have a bad period of an infection in all my tanks in 2018 the worst portion of which lasted 6 months. Supposedly columnaris as suggested by Duanes seeing my dead umbee. If it was columnaris, IDK which strain (of the common 4?) it has been and perhaps is still present in my tanks as I continue to lose fish every now and then. May be not it either. It's very hard to analyze for me with little experience and without involving pathology lab tests.