White Sponges

So it looks like I now have white sponges growing in a few places in my rock. My rock is the fake LR type that is made in California and impregnated with red dye that contains bacteria. It eventually becomes like LR but without hitchikers.

I am wondering how my sponges came. I don't think they could have come from the LR. Could they have come from the LS?

Were they introduced some other way? In general, are sponges good or bad. Some posters on here say they are good because they take silicates out of the water. I am not sure what to think or do.

Thanks as always.