What is happening to this zoa?

[Just John]

I know the picture is terrible, but it's the best I can get. All other polyps look good on this zoa, but this one polyp has shrunk and started turning white, almost slimy looking on one side. It used to be the largest head. I noticed it while feeding them and spot fed it. It closed and ate (I think) but still looked like this when it opened.

 

[mdrieder]

Cant say for sure but if that polyp eventually just turns into grey mush your likely dealing with a zoanthid spider.