Green cup with Creeping White Death

[Spoff4]

Originally posted on general reef disc forum-
I have what I believe is a Green Cup coral with what I will call Creeping White Death. See attached photo.
Any ideas what is causing it, what it is, or how to stop it?

The problem appeared after I moved the coral from one end of the tank to the other while maintaining the same depth.
I also recently lost a Digitata that bleached out over a few days.

Any thoughts as to what is going on?
Thanks.

75 gal 6 year old tank, 30 gal sump and fuge
30 (?) lbs live rock
Reef Octopus skimmer
ATI T5 4x54 (10 month old bulbs)
RODI water, 10 gal water change every 3-4 weeks
Alk 7.7-10
Cal 450
Ph 8.2
Ammonia-0
Nitrites-0
Nitrates- 0

[usmc121581]

I had a yellow cup coral that did that, I could never find out what was happening. So I brought out the dremmal and cut the section out of it. The bad part is that I have a .5 x.5" square in my coral.

[sqwat]

Yikes. I would fray off the dead a little into the healthy part so it doesn't spread also a dip.I didn't no metalica attacked corals.lol

[Neogenocide]

Reminds of some pics I've seen a of nudibranchs attacking a montipora. Can you see any type of bug on it?