Monti missing tissue

[Rwade]

My Monti is missing a bit of tissue in an area surrounded by good tissue. It is on top of the rocks where my Flame Angel does not go so I don’t suspect the Angel being the cause of the missing tissue. Any ideas what would cause tissue loss like this?

 

[Hincapiej4]

Well..imo, it's gonna be either the flame angel or pests..

 

[Rwade]

The Flame Angel tends to stay low, never seen it at the top of the rocks. I guess it could be an Emerald Crab or something I’ve not seen yet.