Yes, definitely HLLE - Head and Lateral Line Erosion. Unfortunately, there is no clear-cut causal path - meaning there is no clear-cut cure. There may well be several different causes.
The most prevalent cause is dietary deficiency. This most often happens in tangs and angels, and anecdotal cures have included greater feeding of "greens" (marine algae), supplementation with HUFAs (like selco or selcon soaks), and supplementation with Vitamin C.
Another reported causal factor has been stray voltage in the tank. This can be easily corrected with the use of a grounding probe.
Good luck.
Kevin