Wierd Growth

[robertfah]

Early today I noticed one of my larger snails (about 2" across) was in a weird position on some of my fake rock. Well later this evening, I noticed the image below and what looks like white discs growing on my fake rock???? I picked one off to look at it closer and whatever it is, it's stuck to the rock pretty good b/c it was hard to get off. They almost look like sacks of some sort maybe holding eggs??? I have no clue.

My tank is a 155g, all water params are at good levels and I the tank inhibatants are:

Fish:
4 Yellow Tangs

Inverts:
1 Bali Star Fish
1 huge Red Serpant Sea Star (about 1 to 2 ft across when it's tenticles are stretched out)
1 Atlantic Red Hermit Crab
1 Fire Shrimp
1 Arrow Crab
2 Emerald Blue Crabs
20 or so misc red & blue-legged crabs.

Any ideas as to what this may be? As I said before, it almost looks like eggs sacks of some type.

Thanks!!

RDF