I'm sure it's algae but you need to post a picture so it can be identified and proper advice can be given.
One thing you can do right now is only run your lighting 6 hours a day. Many times algae growth is caused by running lights too long daily.
Also what size tank, what type of lighting and bulbs are in it, do you know your nitrate level? Is the tank planted, if so what type ferts do you use? Also do you use liquid carbon (Excel) or have CO2?