You really need to take a close look at your tank and what you have done since adding that coral. If you haven't changed anything and it just went south, then maybe it is not as strong a specimen as the others that you received and they are also struggling, but are strong enough to survive. On the other hand, you could have something wrong in your tank (parameter out of whack, predator, irritant, etc.) that got that coral alone.
Like Mike said, frag that one and try to save what you can. Look for anything that has changed (salt mix, water change, addative change, temperature change, lighting, flow, anything that changed...this is why we all keep logs of our tanks...to track what caused good and bad things to happen). Pay careful attention to the other corals to make sure nothing else is happening with them. Look closely both with the lights on, as well as after they go out. Some things happen after the lights go out that you don't normally observe.
I'd double check your water parameters and run some carbon. Keep us posted if you find something that we can look into. Hope this is the only coral that this happens to for you. It's very upsetting...I know from experience.
