Generally speaking how often one scraps the glass is determined by personal preference, but I would guess that most people scrap their viewing glass every day (I do, and everyone that has a SW tank that I know personally also does). Nonviewing glass on the otherhand may get cleaned weekly, monthly or never depending on the person (I personally never clean the nonviewing portion of my tanks). So its kind of up to you - if you don't mind not being able to see in your aquarium except for whatever clear spots you can find - once a week is probably fine.
As far as the green stuff - most algae isn't killed by scraping, its simply removed, so it ends up somewhere else in your tank. If that somewhere happens to be a prefilter, or filter pad where the algae can't survive it dies. If you change these often, no problem, if not, organic debris can build up and add to the nutrient levels of your tank. As far a removing them when you scrap them off, without doing a major water change (like probably 50% to syphon them out, which is never recommended in the SW trade) I really don't know how you would go about removing them. You might try sweeping the tank with a fishnet, and that might get some of it, but otherwise I really don't know.
Hope that helps.
