what sort of water are you using for you top-ups? do you test for alkalinity, magnesium and iodine? those could all be factors here! are any other corals/fish showing any signs of stress?
did it fall to get down to the sand bed, or did you move it. if it fell it could have been caused by the fall. or it could also be a secondary infection from shipping damage as its still relativly new.
PS you should put your tank stats on your profile so people dont have to ask the easy questions like
tank size
lighting (pc alone doesnt mean much saying 2x65 watt pc is way more helpfull)
tank age (it helps early on)
and any periphial equipment your running (if you can fit them in the slots allotted)