Advice? Is Favia Bleaching?

Have had this green favia for at least 3 months. Just started noticing these white patchy areas of zooxanthelle loss. It is the only coral doing this. It is on the bottom under 4 39 watt t5 bulbs in a 40 breeder. My parameters are as follows:
Salinity 1.024
Temp. 76 degrees
Alk 10 dkh
Calcium 420ppm
Ph 8.0
Nitrates 0ppm
Phosphates .25ppm
Nitrite 0ppm
Ammonia 0ppm
Magnesium- Don't know haven't got test kit yet.
Running a reef octopus HOB skimmer, emperor 400 HOB filter and a 9 watt coralife UV sterilizer.
I have 50+ pounds live rock and 2 powerheads a maxi jet 1200 and a hydor 750.
Don't dose alk or calcium because salt mix has kept my tank at the levels above. I use instant ocean reef crystals.
Wanting to know if anyone knows what's causing this issue it has me concerned. Thanks!

 

Here is a pic of corals that are around it.

 

I do my other lps who react when lights are on or in the morning with daylight from room only. The favia has always put its polyps out after I've gone to bed checked one night. Hard to feed when your half a sleep lol. Still will try. I also was wondering I trickle the top off water on that side alot could that maybe be it? Thanks any other advice welcome.