Hollywood Stunner Receding

[MT_is_a_Pisces]

What happened to my Hollywood stunner?
The one on the left was placed in the tank a month prior.
The one on the right was growing and doing great. I went out of town Monday (visit @WWC - superstore) and come back Thursday to most of the coral gone.

Alkalinity- 8.3
Ca - 460
Mg - 1380

 

[Saltyreef]

Ouch.
Any other changes made recently? Is this a different strain from another aquarium?

I had a 6 year old dinner plate size cupped piece melt on me a while back.
Saved only a small portion but i ended up rehoming it to save it.

 

[MT_is_a_Pisces]

Nothing has changed.
Should be the same species. Purchased drom same online retailer.