Bizzle,
I have a Pipe Organ, and I have never noticed sacks. What I do see quite frequently are other organisms growing on the under side of the skeleton.
Mostly these other organisms are q-tip sponges, but every now and then something else appears. I think that the sack is probably not related to the coral, but rather that another organism is using the protection of the pipe organ to grow...and I would guess it is some type of sponge.
HTH
Stewart