Many pond owners already have a dryer duct cleaning kit for cleaning bottom drains, which typically comes with a 2" and 4" brush. Coincidently, there are "string algae brushes" sold that appear to have exactly the same 2" brush, and since I don't use mine, wondered how well it would work, suspecting that it would be a real pain to clean.
A quick experiment showed that it does indeed collect string algae very well - and is a real pain to clean. It took so long to clean that I gave up and sat it aside, hoping that the algae would dry out and crumble off. So in conclusion, while it "works", it takes so long to clean that it's practically a one-time use tool, or plan on letting it full dry between uses and hope you can get it all out of the bristles.
What I ended up using was a wood dowel in a cordless drill, wood because it's got a very slightly rough surface that catches the hairs of the string algae. It ends up with a huge ball on the end of it which can be hard to slide off. A utility knife works well to "unzip" the mess.