Is there something eating my Acan??!!!

[Emma01]

Is there something eating my Acan head? I’ve Come home to this, all was fine yesterday. All other heads are puffed up nicely with no issues that I can see..... yet :-( is something eating it or has the tissue just receded?
Hope someone can help.
Thanks

Parameters :-
PH (redsea) -8.2
KH (salifert) - 8.7
Calcium(salifert)- 400-410
Magnesium (salifert)- 1275
Nitrites (redsea)- 0
Nitrates(redsea)-5-10ppm
Ammonia(redsea)- 0
Phos (salifert) - 0
Salinity- 1025

 

[Crabs McJones]

What do you have for livestock in the tank? Crabs fish shrimp ect....

 

[Emma01]

What do you have for livestock in the tank? Crabs fish shrimp ect....

Hi. I have just one cleaner shrimp in there the ‘ lysmata’ type. But I’ve never seen him even climb onto the acan, so I don’t know?
No crabs. 1 precula clown. And lots of bristle starfish (hitchickers)

 

[timrocks311]

Looks like a possibility.