Video works now. That is weird looking! Just the one spot? Maybe something leftover from external gills? If so likely would be both sides... Fish eating, showing any distress?
Well you know me... when I saw the 'what is it?' topic and the pictures had big red arrows, I was going to say someone 'tagged' your fish. I hate fish vandalism!
My weeksii had external gills forever and they are pretty much known for that. Most endli seem to loose that pretty quickly but that pic has a strange symmetrical look about it.
Only other thing is maybe looks like is a skin fluke, hard to find any good pics of them online. But you are running the fish in qt and doing a parasite treatment so thats good. You might just run that a bit longer than usual just to make sure.