Your birdnest is dying (bleaching) and from my experience, you need to frag out whatever piece that's alive and throw away the white part. It will spread and kill the rest of your coral.
What lights is it on now? What light was it in the LFS? Did you acclimated properly? Temp, ph, salinity and lights in general.
I suggest you bring up your ph using kalkwasher if you are not doing so already and do it steady. 7.8 you are on the slight low side.
also put it towards the bottom and let the corals adjust to the lights.
It sucks but it's unfortunate part of this hobby.