Is this just a build up of film algae? I normally scrape off the front and sides of my tank every 3-4 days(I have never scraped the back) or is the start of some really bad GHA? I have a few turbo snails that will hang out on the back panel but they dont seem to eat it. I plan on doing a large water change this weekend, should I scrape this off during that time or let it be? The appearance doesn't necessarily bother me, I just don't want an outbreak of bad algae. Thanks!