What is it?

Butch 36

Reefing newb
I noticed about a couple of weeks ago,after I moved some rocks around.That I had something growing out of my live rock.It looks like a moth cacoon with looks like a feather duster at the tip...It's about 1/2 to 3/4 in. long ,about the roundness of a pencil..Well this morning ,what ever it was, must have hatched because the sides were broken like something came out of it..I just thought it was some kind of feather duster ,till this morning.