Why is this coral dying?

[MikeP]

Thanks for the reply's everyone. Parameters are all good, I've been battling low alk for a while but it's never below 7 dkh. There is nothing in the area that could sting it. I upgraded lights from 175w MH to 250w MH but that was about a month ago. Could it take that long to see adverse affects? All other corals are doing fine. It was doing very well and had grown from 4 heads to 15 and then over the last 48hrs this has happened.