What is this cocoon thing growing on my button polyps?

[SauceSBC]

I have noticed this strange cocoon looking growth on my button polyps. Its almost a web with a sack, with a long string between it- and it seems to be spreading. Today I noticed it was covering the head of a button, 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 a piece I picked it off.