Green spot algae and green dust algae are completely different. It is GDA that can be allowed to live out its natural life cycle before cleaning it off the glass. GSA forms small circlular spots on the glass and leaves, which grow to much larger sizes. Those need to be scraped off the glass, and if the amount of phosphate dosage is increased it will often stop it from coming back. I'm curious about the other method too? A blow torch???