You know, only now I found out these guys are called Hippo Tangs... I always thought it was just blue tang, or regal tang.
In either case, that sounds like normal blue tang behaviour (I have a small (1.5") in my aquarium.) He's been in the aquarium for around 3 weeks now... he sleeps in the rocks, on his side, with only his eye peeping out. It's how they stay safe at night it seems. They like the comfort of the rocks. Don't worry about hermits or shrimp trying to feed on him either, I haven't had any problems.
Give him a few days and he should be out and about, especially during feeding time. Note that in the future it's always a good idea to ask your LFS
(big al's?) how long they've had the fish for, ask what they're feeding it, and ask them to show you it's eating... they get asked these questions all the time, and will usually have food nearby to show you they're eating.
Blue tangs have tiny mouths, and I find they don't go too much for the pellets, and will not attempt to go for most foods that are bigger than their mouth, even if it's seaweed. In the wild they eat tiny itty bitty bits of algae floating around.
My tang wouldn't eat at first either. I filled up a small mug with aquarium water, then added a tiny (1" square) piece of seaweed to it. I mixed it up for a good 5 minutes with a stir-stick. The seaweed broke down to tiny pieces of seaweed very closely resembling algae.
Only took a few seconds after pouring that into the water for the tang to finally come out and eat the tiny bits of green floating around... everyone feasted for a few minutes. Just be careful not to put in too much of the water at a time, you don't want to pollute the water either.. the bits of seaweed can get into your liverock and rot away.
Hope this helps...