I use a swimming pool leaf skimmer with a telescoping handle I got at Home Depot:

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Aluminum Leaf Skimmer . . . . . . . . . 15ft Aluminum Telescoping Handle

What I like about it is that I can reach any place in my large pond from a few spots around the sides. I scoop the string algae up and dump it in 5 gallon buckets. I never have to touch the slime. Then I take the bucket to my compost pile and dump them. I also use it for azolla and leaves.

Sorry, the Home Depot links don't work.

The Poolmaster Aluminum Leaf Skimmer is Model 61190W

The Poolmaster 15ft Aluminum Telescoping Handle is Model 61207W

Here is a link that might be useful: Home Depot

Thanks rhodyman. I have a leaf skimmer also but it doesn't work for getting the really sticky algae that clings to my two waterfalls. I would like to clean them of that also and this algae witch is supposed to do that like a toilet brush does.
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Here is a link that might be useful: My pond the way it was before the freeze.

Looks just like a giant pipe cleaner. I bet your local hardware could find something for you. Your pet store should have pipe cleaners for plumbing.

OR a chimney sweep brush. Chim chimney chim chim cher roo.

I'm sure that there is a small brush you could use and even possibly cut it so it is narrow or shorten the bristles.

I have used regular leaf rakes upside down for grabbing the Mean Grean hair algae from my plants. Easy on the back. Easy on the plants.

Yes, I use it. It works pretty well. BUT there are plastic parts that eventually wear out. The brush is essential a test tube cleaner on a wire handle. That will eventually break too.

I have brought a Test Tube brush cleaners to replace it (you need an allen wrench.). The broken plastic parts aren't essential but nice. It doesn't seem possible to buy replacement parts, but I suspect a clever person could use a 3D printer to make replacement pieces.

I have also added a screw to the end of the handle and then use a screw gun to spin the tool. It's a lot faster.

The brush used to clean out a dryer hose is extendable and might work for this.