Stress

[Dale]

Hi Kevin.
Your tang could be experiencing stress for many reasons, some might be beyond your ability to detect (ammonia damage via shipping, internal parasites, severe mental distress etc...)
A couple of things to check first though:
pH
Salinity
Nitrates
Bully fish

After that think about your rock work. Does it provide ample spaces for the fish to swim behind and "feel" sheltered? If there isn't then the fish may need to flatten against the rock or jam itself into crevices to gain a feeling of safety. It then only has two choices - swim in the open unprotected or jam itself in the rock. Providing arches and back routes allows for a third option - sheltered swimming.

Next, you could try putting a screen in front of the tank, like big plants or something. Most fish become accustomed to looking out the glass at us but it must be a shock to some initially. When we approach the tank and peer in perhaps we look like large predators closing in. If I were a Tang I'd hide in the rocks too.

(This last ones just a guess)
You might try putting something on top of the tank too. I don't know about SW but many FW fish fear predators from above (birds) more than the side. I've noticed a dramatic change in behavior when I floated plants or driftwood on the surface of my FW tanks.
I don't know if any of this helps, just some thoughts.
Good luck with the Tang