Algae encrusted on back glass

[goodk]

I good scrape it of but no way of capturing the algae as it is scraped off. Is there no chemical i can use,it is only on my back glass and no where else.

 

[twilliard]

I good scrape it of but no way of capturing the algae as it is scraped off. Is there no chemical i can use,it is only on my back glass and no where else.

The way I have always done this to capture the algae is to use a siphon hose with the scraper
As it scrapes it will pick up the algae