I had this same problem. What I had to do to get rid of the problem was the following. Kind of a pain.
1) If problem occurred after adding LR, remove this as it probably contained an abundance of phosphates. This site offers an excellent selection of LR.
2) Shut off your pumps, wait for overflow to stop, and use a razor blade to scrape off the algae from the walls (take care to remove as much as possible while scraping)
3) Using your siphon, perform a 25% water change (RO Water) - try to siphon off as much of the lose algae as possible.
4) Over next few days ensure to rinse off any pre-filters in your set-up as these will gather the lose algae and produce nitrates to feed the algae.
Note: Only use RO water, as some water sources contain high quantities of phosphates.