Acan lossing/receding head

[steallife904]

I have 4 acans (each has 6-10 heads) in my tank. They have been in for months. They all have fully open heads and look great but one of them seems to have a head right on the edge of it receding down to where I can see skeleton. All other polyps look fine. This acan has actually already started to encrust on the rock as well. What could be the issue? My parameters have not changed, I target feed once a week and feed frozen to tank 4 times a week. Even dose a little strontium from time to time. Any idea?
Alk is at 8 and calc 400-410. Cant detect nitrate and phosphate but its always been that way.

 

[Uncle99]

If you can’t detect Nitrates or Phosphate, it’s too low, nitrates should be in the 2-5 ppm range and phosphate in the 0.02 to a max of .1.

Otherwise LPS slowly die, regardless of manual feeds