I found the best thing to clean the string algae out of my backyard pond--a shrub rake from WM. It has an 8" plastic head on a 4' wooden pole and cost $5.94. The plastic head can be scraped along the pond sides or used as a skimmer on the pond surface, then a brisk shake dislodges the collected algae into a bucket for the trash or compost pile. The head is soft and flexible so won't damage the rubber liner or the pond plants.

If your pond is larger than 8' across, you could attach the head to a longer pole.

I have also tried soft rakes. They do well. But when the water is warm, I kind of like wading in it to get the algae further out. My back won't take the bending over and over. And I can get up close and personal with my fish. Silly things.