To be honest, it's very very common in the hobby. Obviously no one spends $2,000 and is happy about anything but a flawless specimen but I would venture to bet nearly all zebrasoma tangs (yellow, scopus, Sailfin, especially purple) have some degree of HLLE prior to arrival. I think in this case because this is far more of an investment it is more concerning or noticed whereas on other tangs it might not be.
IMO, poor diet, water quality, and even stress contribute heavily to this condition.
The good news is that zebrasoma tangs are very hardy, this guy looks otherwise examplary to me, and he will recover quickly from what appears to be minor HLLE. My zebrasoma tangs exhibit more lateral line than many other species, but especially when they have been treated with medications, but I have no data to back that up I just notice it more under these circumstances. It does get better quickly once through quarantine and treatment process.
Lots of varied types of nori (green and red are my absolute favorite - emphasis on red), soaking foods in selcon and a dash of cod liver oil over time mixed with healthy food offerings will help and keep it healthy and happy.
Nice fish by the way!