Scrape it off or get an algae eater of some kind (ottos are great for that). Make sure your lightbulb is less than 10,000 degrees Kelvin, that helps too. And keep natural sunlight away.
If you have fake plants get like 1 snail for every 10g and your tank will shine in a week. Also you could get a Pleco if tanks big enough. Snails do a better Job on tanks with full alge. Plecos do better with alge here and there.