I would like to hear more about your tank, personally.
What size, depth is the tank?
What is the wattage of the MH
and what distance from the surface is the light, what color temp?
Any other supplemental lighting?
Did you 'light acclimate' it properly, that is cut back on the lighting or keep it in a spot of low light for a couple days and introduce slowly to your lighting?
Is it time for new lights? : )
How long has the tank been up and running (Cycled)?
What are the params such as NO2
? What is the SG
Is the temp stable (Not many are without chillers this time of year...)?
Have you offered/accepted any food?
Was the coral healthy when you purchased it (ie - no tears, was 'inflated' and normal in color)?
Has it bleached any?
Is there any direct flow on the coral (ie a PH that is pointed directly at it, or perhaps no flow at all)?
Do you have a sandy bottom tank, bare, etc?
Any known predators in the tank?
Just a few off the top of my head....