pond scum

American  

noun

1. any free-floating freshwater alga that forms a green scum on water.

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pond scum British  

noun

1. a greenish layer floating on the surface of stagnant waters, consisting of various freshwater algae

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Etymology

Origin of pond scum

First recorded in 1885–90

Example Sentences

B. maeuseri presumably got its dinner in a very similar way, but also more flamingo-like, rippling tentatively through the shallows, slurping up mouthfuls of lagoon water and filtering out the pond scum.

From Salon

At those Olympics, everyone knew about the four-man crash and the pond scum.

From Washington Post

On Earth, such sediments have preserved evidence of ancient life in the form of fossilized mats of microscopic pond scum called stromatolites.

From Washington Post

Microbes and larger multicellular organisms covered much of the seafloor in an organic mat similar to pond scum.

From Scientific American

Green pond scum floating on a lake is not just unsightly.

From Seattle Times