Test your water for phosphates and also nitrates. Also, make sure that the tank is not getting any direct sunlight from outside as the increased lighting (including the lights that are on your tank) can aid in algae growth. Algaes feed off of nutrients in the water (phosphates and nitrates) so this is most likely what is going on within the tank. If this is the case, we need to figure out where the nutrients are coming from. Are you using tap water? What, how much, and how often are you feeding? How many fish do you have? What size tank is this?