There are several similar-looking species of yucky pond-infesting green algae that can only be distinguished with a microscope.
Spirogyra can be recognized because its chloroplasts are arranged in helices:
But Zygnema has star-shaped chloroplasts:
And Mougeotia has ribbon shapes:
To the the naked eye they all look like slimy green mats.
This is on my mind because over the weekend Toph and I camped in Parvin State Park, which boasts a swimming area in their lake. The swimming was not good, in large part because the lake is filled with slimy green mats.
The Protist Information Server at Hosei University is the source of the Mougeotia image above, and has many additional images of all sorts of yucky green slime. The Spirogyra image and Zygnema image are from Wikimedia Commons; Zygnema is by Bob Blalock of English Wikipedia.
This field guide to scum, by M.S. Kannan and N. Lenca of Northern Kentucky University, was comprehensive and interesting.