Back in the 90's, I used to see many fish with severe HLLE, mostly in FO tanks, often medicated to an inch of their lives. Blue tangs were notorious for the condition. It is quite possible for a blue tang to lose flesh/color, and have "melting" fins to the point of almost no tissue, with ZERO aggression from other fish. The pictures in this thread are EXACTLY as I remember what I am describing....the "webbed" look to the remaining tissue at the front of the dorsal fin and the split/separating/fraying tail tips are classic, IME.
The condition, however, does not happen over night.
On a side note, I'm not sure the melting fins and HLLE are one and the same, but they always seemed to accompany one another....