Not sure what this is.

[Girthrockwel]

See the three little domes on the rock and there's a lone one above them. I'm almost positive they're new. I just noticed them today. I poked at one with a toothpick and it sort of crushed one side almost like thin rock like paper thin. These little bumps are small, maybe 1/16". Here's a picture after I poked at it. Also before anyone comes down on me for poking, I know I shouldn't have.

 

[Girthrockwel]

Ah crap, it posted my pictures in reverse order. The top picture is post poking.

 

[Rick cowen]

Possible sponge

 

[Girthrockwel]

They seem to be multiplying. I'm counting 9 on the portion of the rock that I can see. I got a little better picture of some of them. One of the snails is pointing its pointy end at a dome.

 

[Shaddow_wolf169]

Sponge or egg sac

 

[Tahoe61]

Nerite snail eggs is a possibility, if you have Nerite snails.

 

[Girthrockwel]

I do. Man, if that's the case snails are some egg laying critters. I think it's cool that things are trying to procreate in my tank. I've got a whole ecosystem going.

 

[Maritimer]

Nerites lay a lot. They're cool enough on the rocks, less so on the glass.

~Bruce