2nd time guessing

[Gibb72]

Have this growing on the bottom of rock, 2nd one has started on other side of rock. Now have a 2nd rock with it growing and bottom or rock gone all white. Don't have a clue what it is

 

[czwicky]

I dont know but if I was you I would get rid of those rocks. Doesnt look good IMO.

 

[Supa Belle]

Looks like sponge