I battled red slime in my 10g for months. there are quite a few factors that could be the cause. run a search on red slime or cyano bacteria there are some good threads on how to rid your tank of this pesky stuff.
in my case I got it under control by: doing frequent water changes, swiching from DI to RO/DI water, cutting my lights back from 10 to 8 hours, and last but not least I got a turkey baster and manually removed as much as I could. also I don't know how much you feed but I only feed a half of a cube of formula1 every other day, and i have a clown, a 6line, and lots of shrimp and crabs.