Don't know what this is??? Help!

[kbmlpn]

These are white/off white, round growths that are appearing on my live rocks. This is an established tank with no new additions in many months. The center appears to have a small opening and the edges appear to have small, thread-like appendages.
Picture quality is not great, I have tried with camera phone and digital camera, can't get any clearer.

Any suggestions?

 

[Rutrag]

My guess is it's some sort of sponge.

 

[kbmlpn]

Thanks!

Any idea why they would just appear with no new additions to the tank?

 

[rdnelson99]

If it is a sponge, I have been told that it is a sign of a very healthy tank. I saw something moving around my sand bed a while back and a couple people said it was a sponge and that I must have a very stable and healthy tank.