I have seen MANY eat algae, specifically macro algae, seaweed, and nori.
If that is all you are feeding the tang, that is why he has HLLE. I would recommed using seaweed sheets, frozen cubes of kelp, algae formula, and formula A and B to offer a large variety in the fish's diet. Every fish really needs to have 4 to 5 different types of food in their diet.
Using vitamins on ALL the fish's foods such as Zoe, Selcon, and VitaChem will quickly turn around the HLLE as well.