Do you mean above the substrate, or just below the top of the substrate?
The latter is natural, diatoms or something very similar; the same thing gets in filter hoses sometimes. If you push the substrate back this can easily be scraped off, but I never bother.
If you mean above the substrate, it is algae and can be removed with a scraper. I periodically use a hard scraper along the front and push it just down to the substrate, being careful not to pick up any grains of sand or gravel (which is very easily done) which will then scratch the glass.