SPS Tissue loss?

[RCFA]

I am wondering if anyone can help me out. One of my corals(I think it is a type of porites) has slowly (2 weeks) been losing tissue. The polyps have been pulled in most of the time but not always and it looks like the dead spot is growing. I don't think this is RTN because it is happening so slowly. I though it might be a flow issue so I tried putting it in a different spot to see if that helped, but I am wondering if it is too late. All the other coral look great, continue to grow and have decent polyp extension. Hopefully these pictures work, this is my first time uploading photos. nothing really that could have stung it either







As far as tank parameters that I know:
Ammonia, nitrites: 0
Nitrates: <5
Ca: 390
salinity: 1.024
temp:78
pH:8.3
alk: 10

Any ideas how to save this piece? should I try fragging the unaffected parts? or will it recover if the conditions are better?