Sounds like bga a type of cyano..see red slime too. You can increase water flow and check the system for a nutirient source. Bulbs sometimes need to be changed as well. If you are using tap water this could be a source of the nutrients as well as overfeeding and even marine type water supplements. . The fixes for bga are the same as the fixes for red slime.
Identify the source , make the appropriate changes and the problem will go away. DO NOT USE and miracle additives as the problem will only come back. Also, massive bga or red slime die-off CAN..not WILL .... cause water quality issues and toxin issues leading to a total system wipe out.