What are these? Eggs, snails or what?

[Reefifier]

I found these on the bottom of a ceramic plug rack that was on the sand.
You can see a frag plug hole in one of the pictures.
There seems to be a dark spot inside, it's not moving and one popped when I tried to flich it off

 

[Timfish]

Could be, are they moving around or staying in one place. If they're moving from day to day that rules out a sponge of some kind.