Looks like mulm. Some say " MULM??- What is mulm" ?
It is a bacterial slime which coats the sand bed and other surface areas. Mulm is also a combination of algae, sponges, bacteria, pods, worms, detritus, poop and any thing else that can be propagated or grown in darker or dead zone areas. I realize most people would immediately get out the sponge, razor blade or grenade to remove it but there is a word I like to use to describe those people. . . . slow down !. Mulm is a natural product that you will find in the sea all over the world. Our tanks run on bacteria, algae and a food chain. Bacteria and a food chain are dependent on having a place to reproduce. Mulm is the perfect place. Rocks and glass are flat surfaces that are only two dimensional. Mulm makes these places three dimensional allowing much more space for bacteria and microscopic organisms to grow. It looks aweful but is also harmless because the beneficial bacteria love all of the organic wastes that have accumulated on the concentrated substance.
Admittingly, I remove it as I see it, but there is no great panic. Would I clean some of it up- Yes, and increase water flow to reduce the buildup if desired