Growth on live rock

[Ruth]

There is a growth on my live rock in my 12g. Nano tank that I have at work. I don't have a picture of it cause my camera is at home and not work. It is shaped like a barnicle but it casts out an almost spider web like thing that you can then see it reeling in. Once it has done reeling it in it is quite common to see it spitting out what it doesn't want. Anyone tell me what this thing is? It is about 1/2cm. round and sticks out of the rock about the same distance. It started out even smaller but has grown to this current size.