The whiteness looks like what my South American Lung went through. He acted listless and wasn't eating. After a week I saw slight deterioration of his tail fin. I proceeded with Seachem Paraguard, and he started eating again a few days later. I had to move to something stronger once I noticed more fin deterioration (still was eating and had begun acting normal), and used Seachem Polyguard.
At a minimum, I would use a mild medication like Seachem Paraguard for 1 week and see if there are any changes to the wound. It's dosed daily. My scaleless SAL had no problem at full dosage.