I used to use a plastic scraper designed for acrylic tanks, but I think it was still scratching my tank. The tank bows a little when filled so the edges struck the tank harder than the center. I read about someone using an old t-shirt to clean the algae off the inside of their acrylic tank on these forums and tried using a small microfiber washcloth instead. It works SO much better than the scraper, even with stubborn spot algae. Just make sure the cloth is rinsed well to remove any detergent residue and only use for your tank.
I highly recommend trying it.