I agree -- it's HLLE, which is really common in tangs, and really common to see around the eyes.
Do you feed your tang vegetable matter? It is an herbivorous fish, and the majority of its diet should be foods like dried seaweed sheets and plant-based frozen foods like Emerald Entree. Meat-based foods are fine, and tangs will eat them happily, but it cannot survive on meat-based foods alone.
Stray voltage has also been implicated in causing HLLE. You may want to read this and test for it:
Identifying, Locating and Curing Stray Voltage in a Saltwater Aquarium
Also, some people believe that running carbon filter media can lead to HLLE, so if you are using carbon, you may want to reconsider.