What is this?

I can’t tell if this is a bacterial growth or not. I added this to my tank a few weeks ago and just noticed these things on the underside, not on top. Just the bottom.

It’s fallen a few times so i thought it was sand at first but it looks like it has stalks. I’ve removed it from the tank and I don’t see any anywhere else. Yall think this is going to be a problem? Is it what I think it is?