Tangs can be territorial with new additions, but with other fish. I never heard of a tang attacking a star before, but in any case as long as you have plenty of dark hidding places in the rockwork the star it should be fine. Out of sight out of mind for the tang.
If the star wasn't acclimated properly they can sometime start to melt down and the tang may have been picking to sort of "help" it along either by dropping a leg or what not. They aren't normally carnivores but they aren't going to turn away a free meal either.
If both were 100% healthy I'd expect them to leave each other alone. But if one or the other was ill, I'd expect them to both take advantage of the situation.
The star was in the tank a month before the tang was added but I suspect you are right about the star not being in the best of health. His body color has been fading out recently. Mr Tang was probally grabing an easy meal. I have moved the star to a hospital tank to see if he can recover.
Jeff, my PBT is pretty aggressive. I have a pair of ocellaris clowns that live in some xenia. When the PBT is mad (territorial aggresion) at the clowns, he takes a small nip out of the xenia. Kid of funny and no harm done
