What is this?

[Toolfan333]

I picked up a zoa frag a few weeks ago with some orange and green on the same plug. I noticed this thing growing from the center a couple days ago and around the time I noticed this, only the orange stopped opening. The green seem uneffected.
Any ideas what it is?

 

[Doctorgori]

tentatively looks like a nudibranch but I only answered 1st to win the prize… honestly not sure

 

[Toolfan333]

Anyone else?