Sponge. I think they are called mystic sponges. I have those in my overflow. At first they look white, but as they get bigger they are blue. They dont seem to get out in the light, they seem to be under rocks, ledges or in overflow on the glass and standpipes.
It does not seems to move when I touch it with fider stick. However it looks alot like a spaghetty warm. It is white with long white tentickles growing out of it. It grows into something reseblimg a spider web. The main part of it sits inside the cavity of the rock.