Red algae and cyano are the same thing. It's a bacteria that grows in sheets. Mostly due to excess nutrients in the water. This would be phosphates and nitrates. Even with a phosphate test, you would get false readings. Cyano holds phospates, and the reading would be lower then it truly is. First thing is to attempt to remove as much of it as possible from the sand and rocks. Second, lower the amount of food fed or cut back the number of times fed. This will help lower nutrient levels. You should also cut back the amount of time the lights are on. Nutrients is the cause of cyano/red algae. There are tons of threads about cyano, cause and possible cures.