Looks like java moss. Ice got a patch I thought I also got rid of. Lol. And I usually take out half of the substrate and change it when it's needed. The breakdown to mud is happening in my tanks too. But I have noticed it helps boast the plants growth as I don't douse chemicals for that.Yes it's moss. I thought I had removed all of it last year but missed a few strands. I'm eventually change out substrate as its breaking down to mud.