Plate Coral dying?

[martym]

I think my plate coral is dying There is a section of it that doesn't inflate and has turned white.
Calcium 460
Ph 8.0
Sg 1.0235
temp 80
ligths 2 daylight, 1 50/50,1 actinic all NO
Nitrate 5ppm
0 phosphate
Kh dipped down to 80
I add Stontium and iodine and reef complete.
I got it from Jeannette and Ian, it was thier first coral, so it is about 1 year old.
Any ideas, and is it possible to bring it back to former glory?