I in for a vote on a sponge (for the smooth white gizmo). I speculate that they may be more common in new tanks. I recently started a 10G nano and took some rock from a 20 year old tank - I noticed something like that on on of those rocks this week. The curious part is that I didn't see them when they were in the other tank. I suppose it could have come from the cup of live sand I got from the LFS, but they did not show up on the rock that came from the same place. Boggle.