Duncan help. Heads not coming out

[currentking]

I have a Duncan in my tank that is in the corner pretty much alone and been there for about 6 weeks extremely happy. Polyps expanded very large. 4 days ago half of the colony has not come out at all. The other half has been out very happy. Perimeters have not really changed and flow has not changed. I have an emerald crab that lives in it but it hasn't harmed it in the past. There is no mucus or anything. I could understand if the entire colony did this then I would worry about water quality but it's half.
Any ideas? What are the chances it will survive?

 

[Tab28]

That does look odd. But since it is the portion facing the front glass. Could it be current? Is it bouncing off glass differently?

 

[currentking]

Current has never changed. That was the first think I looked at. I repositioned yesterday and has not helped.

 

[REEFDONKEY]

looks like it caught some nutrients in the water flow. happens alot.

 

[REEFDONKEY]

or emrald irritating around that area