Similar to what Blucat said, I have a algae cleaner from Petco which is just a stick with a sponge on one end and a thin, wedged plastic piece on the other end. It works great on everything that I've used it with, both glass and acrylic. The plastic end can probably scratch the acrylic, but the sponge end works great. The plastic end works great on getting green spot algae off of glass.
I think it cost less than $3.