What is this cocoon thing growing on my zoa??

[SauceSBC]

I have noticed this strange cocoon looking growth on my zoa. Its almost a sack, with a long string between it- and it seems to be spreading. Today I noticed it was covering the head of a zoa piece, so I picked it off. To my surprise, the part covering the head was kind of hard- like a dried out piece of abc bubble gum.

I'm not sure if this is how this particular species spreads- but if that were the case, I don't understand why it is growing/spreading over itself.

Please help me to ID. I am attaching pics, plus a pic of the piece I picked off.

I don't think this is a peanut worm. No movement or retraction when I touch it- just spreading slowly. Sponge? Algae strain? I brought the piece I picked off to my local LFS. He cut it open, and the middle was very solid. He (The store owner) wasn't sure what it was- I was hoping I could find my answer here.
Is this cause for concern? Should I remove this? Is this potentially harmful?

 

[SushiGirl]

Looks like sponge.

 

[cbrennan4]

+1 on sponge

 

[Sugar Magnolia]

Agree with the others. From my experience that type of sponge can be kind of invasive in a nutrient rich tank. Doing routine maintenence to keep the nutrients down will slow the growth. Pull off any that encroach on corals.

 

[dadummy]

zoa boogie?

 

[JMCAquarium]

zoa boogie?

lol

X3 sponge happens alot in zoa colonies

 

[A. Grandis]

Yep, sponge.

Grandis.