Fish disease diagnosis is very difficult. Some things that stood out to me-
The patches on the skin
Doesn't have a sunken belly, but not really eating aggressively
No aggression from tank mates
I don't think it is HLLE. That would creat more of a discolored pitting, and generally shows the most around the face and lateral line.
If you don't see ick, or any other external parasite, I am leaning toward internal.
Bacterial infections are usually red/swollen
Fungal infections- It doesn't look like a fungal infection.
I am guessing internal