Last week, I posted a pic of brown sand that I thought might be related to a feeder block as it was only in that area. General consensus was that it was diatoms and would go away on its own. They have cleared up mostly and now a green film is starting to grow on sand in those places. It's made the top layer of sand thick and chunked together. When I tried to do my weekly water change this morning, I couldn't get the sand clean because the poop and plant matter that the fish kick up was stuck in the chunks of sand. The green is subtle, but bright and just starting to show up/spread. Is THAT cyano?