It's caused by phosphates in the water. Lack of flow allows it to stay there and grow.
I have a powerhead on the right side of my tank near the top. I have it pointing at the front of the tank and just barely, slightly pointing downwards and slightly to the left, (can post a pic if you want to see). Anyway, the flow hits the front of the tank and flows down it, to the sand and towards the left, thereby pushing a mild flow of water from the right side of the tank to the left, across the sand on the bottom. This has helped along with less, much less, feeding and lots of water changes to get nitrates and phosphates down to very low levels. I'm doing much better now with cyano