Without a picture I need to guess that it is green dust algae. The "film" description implies that is what it is. This stuff is actually a zoo-spore, you scrape or wipe it off...it swims around and is reattaching itself to the glass in 20 min.
Algae eating critters like "Ottos, Snails, etc." will eat it but it is tough to get ahead of. Scraping with a blade will give you relief for a few days.
There are two ways to defeat it.
1. A 72 hour complete tank black out. That's the wrap the tank in opaque plastic trash bags with NO light for 72 hours and you will wipe it out. It may come back in a few months.
2. The best way is to leave it alone
. Let it grow and block out your sight into the tank. I let mine go for 3 weeks until it was so thick it started to peel off by itself. At that point it easily scrapes off and is vacuumed and filtered out with ease. What happens is you have let it almost finish out its life cycle and it changes form. Once you have reached that stage...scraped and vacuumed out and done a water change you will not see it again for years.
Jay
