it means your tank is quite normal, it's just a type of algae that likes to settle on the bacterial film on the glass. Not much you can do to prevent it, but turbo snails do like to eat that stuff and IME will keep it until control.
Well, it shouldn't be really heavy or even that noticable on a "DAILY" basis. Might be a sign of too many nutrients or something. You have algae problems anywhere else. Other than that it is NORMAL. I'd say every 4-7 days I have to clean mine. 1-2 times/week. THe stuff you rub off will go back into the water for filter feeders too.