Any ideas

[CoryDude]

I have an orange montipora digitata that has done great for the past 3 months. But, it's begun to have some tissue recession on a lower branch and near the base. The polyps do not extend near the base and now the white skeleton is starting to show through. No other problems have been detected on other inhabitants.

I've checked for nudibranches but haven't seen any evidence of egg sacks or the creatures themselves.

Here's the 411 on my 90 gallon tank that's been established for 5 years:
Light: 2X 250W 14K Bluelife and 2X 110W VHO Actinics (yes, I'm still old school and use VHO)
Filtration: 110 lbs of tonga lalo rock & EuroReef CS 6-3
Water movement: Iwaki 40 RLT for sump return w/2 maxi-jet sure flow 1600 mod kits on a red sea wave timer
knop C calcium reactor w/koralith media

H2O Parameters
Salinity 1.025
PH: constant 8.3
Temp: 79-80F (constant temp w/ 1/3 HP chiller)
Alk: 3 meq/L total hardness and 7dKH
Ca: 425 meq/L
Mg: 1325
Phos: <5ppm
Nitrate: 0ppm

Any ideas or personal experiences are GREATLY appreciated. Thanks guys!