I saw this when I was wiping the acrylic down on my aquarium. It’s a freshwater aquarium with some moss on a driftwood bonsai. 2 mystery snails, 2 yoyo loaches, 2 khuli loaches, 1 pleco, 3 dwarf guorami’s, and some glow fish tetra. It looks like a beehive kind of. Gritty texture. I don’t know if anything is mating in there.