Not saying I know what's actually going on, but I'll share my *guess*
My blue hippo loves to hide in the smallest of crevices in my tank. Some of which have sharp or jagged edges that could easily cut flesh. I wonder if this is happening and what you're seeing is healing tissue?
Do you ever see the fish scratch/rub up on things in the tank or hang in areas of high current like it's swimming on a treadmill
Cheers!
PS: FWIW If it belongs in the disease section, you may as well put it there. It's likely that a moderator will move this thread there anyway