This is simply my opinion..
I work in the industry and have dealt with HLLE far to many times... IMHO, it's usually brought on by stress, or a nutritional deficiency. I believe the link between GAC and HLLE is due to tangs ingesting GAC dust. Poison control doctors often give their patience carbon to ingest. The carbon adsorbs the poison along with beneficial nutrients , keeping these substances from entering the bloodstream. The same thing happens when a fish ingests GAC dust. The GAC robs the fish of vital nutrition. GAC maintained within the filtration areas of the aquarium system is harmless. It only becomes a problem when dust from the GAC is allowed to escape the filtration and cover the surfaces within the tank. A tang that is constantly grazing these surfaces will constantly be ingesting carbon. Naturally, this would lead to nutritional deficiencies.
These fish are largely herbivorous, but will eat just about anything. Many hobbyists feed them regular prepared foods for the aquarium hobby. As long as they're eating, many hobbyists assume the fish will be fine. While they will eat this food, most won't provide the needed nutrition for these fish. They need their greens. When these fish don't get an adequate amount of algae in their diet, they often suffer from HLLE.
Stress is also a large contributing factor in HLLE. Just as in humans, stress often shares similar symptoms with malnutrition. Weight loss, compromised immune system, lethargy or behavioral changes, loss of skin pigment, and many other ailments or symptoms. As a tang grows, to it, its enclosure gets smaller. It's stress level increases. Tangs that are fed a good diet, and are not ingesting GAC dust, often still show symptoms of HLLE if kept in tanks that are to small.
I think there should be one more symptom added to HLLE. It's often not just the head and lateral line that's effected. Many individuals, especially from the Zebrasoma genus, will show fin loss. This fin loss is associated with the same contributing conditions, and often show up along with HLLE. I believe head, lateral line, and fin loss are all one in the same. Just different symptoms of the same problem.
Peace
EC
