A lesion on a fish: what could be the cause?

[DaisyDew]

I recently picked up some angelfish from a large online vendor, and two were supposed to be Santa Isabel (P.scalare). The SI arrived in pretty rough shape - severely stunted, which isn't what I expected. One of them has a few flat white lesions on its head, and someone mentioned it could be HITH. I've been trying to research it online, but all the info I find shows advanced cases, so I'm not sure if that's what I'm dealing with. Could there be another explanation for the lesions? I've isolated the two SI for now and am not treating them yet.