A pic would be helpful.
Here are the solutions for (imo, and ime) the most common algae causes:
Buy a magnetic glass scrapper, or a stick scraper.
Cut down on light, if the tank is near a window, buy curtains or blinds, or use the ones you have to minimize sun exposure to your tank.
Lastly, feed less, most fish only need to be fed every 2 or 3 days.