Which one?, the one with the green bulbs on a stalk, or the green/red patches on the sand? If it's the patchy stuff, it's mostly cyanobacteria. Most new tanks go though several stages of algae. If your tank is well established, and the red slime persists, look getting getting some Phosban to reduce your phosphate levels, and increasing water flow isn't a bad idea either. If your light bulbs are old, over 10 mths, that could be a cause as well.