Powder blue's are pretty much at the top of the list as far as hard to keep tangs go. I would not recommend them for new set ups. In fact many don't even acclimate well in established mature tanks.
Given the size of the tank and lack of available places for the tang to hide, I would say it is extremely stressed. Chances are very high in these cases that the animal will not survive. It also won't take long for the dottyback and/or the clowns to take advantage and start harrasing the tang.
Best advice in this situation is to return the tang. I can see no other way to solve this....