Hi Humblefish,Head & Lateral Line Erosion "HLLE":
Symptoms - HLLE literally looks as though the skin is rotting or peeling off the fish. Affected areas usually include the face and lateral line, and may be white in color.
Treatment options - This is more of a condition than an actual disease. Since no one can agree on the exact cause, all you can do is address all of the following theories regarding HLLE:
1) Stray voltage - Use a titanium ground probe & test your tank for stray voltage.
2) Running carbon - Don’t use it, or use only premium grade carbon.
3) Nutritional deficiency - Feed high quality nutritional foods. Some have had success reversing the effects of HLLE by soaking food in “fish vitamins” and also feeding nori.
I have a whitespotted grouper, it hasn't eat any food for three months, but before three months, it has swallow a live fish. Before I treat it with a Seachem Vitality, it has serious rot fin. But now I think it's better.
I'm not sure if this counted as HLLE, it has been like this for a long time.
The top of the head is pretty obvious.