Duncan Coral Help

hello everyone, I belong to a few of the different reefing sites. Wanted to get a few more opinions befor i started dipping and cutting. my duncan has 3 dead spots on it. Want to say it was this way when i saved the tank but i can not be sure. recently rotated the coral and saw this. been told just a dead spot and will be ok... and that its the brown jelly! and I'm in trouble.

STATs
pH-8-8.2
ammonia-0
nitrite-0
niTRATE-20 (in the last month)
phos-0.5
calcium-350
mag-unknown
alk-9
rescued tank 8 month ago, 75 gallon.20 gallon sump, basic skimmer, running activated carbon nitrate and phos sponge.
this is what it she normally looks like