Here's a paragraph from an article I read recently....
"One final interesting note is that because slime algae consume nitrates, often when aquarists perform nitrate tests, the readings come up as normal. Don't be deceived. If you were to remove the algae temporarily before putting into action any of the above solutions, in all likelihood you will see a rise in the nitrate levels in the aquarium. It's like a Catch 22. The nitrates have actually been there all along, but unreadable as the algae is feeding on it, therefore the nitrates appear to be in check. This applies to many other forms of algae as well!"
The full article is here.
http://saltaquarium.about.com/od/diatomandslimemicroalgae/a/redslimealgae_2.htm