a greenish layer floating on the surface of stagnant waters, consisting of various freshwater algae
pond scum in American English
a mass of filamentous algae forming a green scum on the surface of ponds, etc.
pond scum in American English
noun
any free-floating freshwater alga that forms a green scum on water
Word origin
[1885–90]This word is first recorded in the period 1885–90. Other words that entered Englishat around the same time include: foregut, roller coaster, sensitization, toner, upsweep