Use an algea scrapper, magnetic or not, that is safe for the type of glass/acrylic you have, and swipe clean every day. After even a few days, depending on your nutrient levels, this stuff can build up to be too much to simply scrape, and take alot of work to maintain.
I like to combine my glass cleaning, daily, with some other method of daily maintanance. Somthing needed daily, and can be done quick too- like dosing, top off, ect.....takes little time and becomes routine in no time. A week of buildup can be tough, aspecially on an acrylic tank, but take care of the issue daily, and its soooo much easier.
I swipe my glass every 3 days or so now, and it doesnt even need it, but a week later and its tough to clean. I had to clean it every other day- just to see through it clear, untill it went through all its cycles, and I dont mean just the nitrogen cycle. If the tank is new/newish, you will have to clean more often. If the tank is "firmly established" and has excellent water perameters, you can ease up on the glass cleaning significantly.