Acan sketelon exposed through tissue/receding

[newyorkmets15]

Can someone help diagnose what’s happening to my acan. Other corals in tank include torches, hammers, frogspawn, alveopora, and zoas. All open with full polyp extension. Everything seems happy and healthy except acan

Salinity 1.025
ph 8.13
Tmp 79.5
Alk 9.8
Phos .11
Nitrate 16.8

 

[newyorkmets15]

May have found the culprit. Noticed peppermint shrimp picking at my other acan while the lights were off