Acan tissue receding

[Rpc07]

I noticed tonight that one of my acan frags the tissue seems to be receding. My parameters are

 

 

Temp 78-79

Sg 1.024

Alk 9

Ammonia 0

Nitrite 0

Nitrate 5ppm

Phosphate .04

Calcium 380-400

Mag 1280-1300

 

I spot feed them 2-4 times per week and I don't notice any fish or invert harassing them. The frag just next to it is thriving with many new polyps growing. No other corals show any signs of stress

 

 

any help is appreciated.