Check your water parameters. High Nitrate levels will cause algae blooms, and is one of the reasons it's a good idea to do a weekly water change of about 25%.
If you don't have any plants the safest way to fight the algae is to "black out" your tank for about 5 days. Keep the aquarium light off, and cover it with a dark heavy material that won't let light through. No light = no photosynthesis for the algae.
Just be forewarned however that once you have had a serious algae bloom it becomes harder to prevent them. Keeping your nitrates as close to 0 as possible will help though.