Tissue die off on cup coral

[Waco Pilot]

I have a small cup coral frag with about 8 polyps on it. This piece is about 3" id diameter. It is in a higher water flow area near a large cup coral. About a week ago, we noticed a brown spot on it and began watching it. The spot grew to about 1/4". We took it out 5 days ago and dipped it. The brown spot basically went away, however, the tissue started turning white and is now about 1/3 of the way into the coral. 3 polyps are completely gone now leaving basically hollow holes where they once were. At this point, should I attempt to cut this area out of the coral or is it too far gone to save? The rest of the polyps look healthy and are extended.

Ammonia 0
Nitrite 0
Nitrate 0
PH 8.4
SG 1.026
Phosphate .25
Calcium 440
KH mid-range
Temp 80
75g tank
6-light T5

 

[Waco Pilot]

One other thing -- about a week ago -- the turbo snail was "turboing" around the tank at night and knocked this coral over onto a rock full of zoas... in the morning we found it and righted it -- but later that day or the next day is when it started exhibiting the decay. I would "bet" that has something to do with this -- but maybe not. Four polyps are part of the decayed area -- the rest of the polyps are open fine... just don't want this to spread more.

Thanks!