Any idea what caused my Cynarina to shrink and spit loads of stringy mucus?

[Koty]

My Giant Cynarina started to shrink and spit loads of stringy mucus. It came back to itself the next day as if nothing happened:

On the left, this is its normal size:

 

[KrisReef]

Don't know if it was spawning or clearing waste products?

Are you running carbon, incase there is something in the water that is bothering it?

A mystery to me.