As mentioned, dissolved silicates in your water supply feeding diatom growth. Rather common. I breed angelfish, run barebottom tanks through 90% of the tanks I own, do 20% daily and 50% weekly water changes on those tanks and I still have to scrape it off by the time the next 50% WC comes around in a week's time. You may find filter media that will absorb silicates. I've found that using Purigen slows diatom growth significantly, but doesn't eliminate it. Snails love the stuff. By comparison, my 180G planted low-tech has a mix of snails and I haven't scraped that glass since the spring.