pond scum
noun [ U ] uk/ˈpɒnd ˌskʌm/ us/ˈpɑːnd ˌskʌm/
（池塘等水面上漂浮的）绿藻层
small plants that grow on the surface of a small area of water
Scientists claimed that they had found a way to "grow" hydrogen gas from pond scum. 科学家宣称，他们找到了一种让池塘绿藻层 "生长"氢气的方法。
mainly US informal (UK informal usually pond life)无用的人;可鄙的人;平庸之辈
people that you do not like or respect at all
He is an odious little piece of pond scum. 他就是个可恶的人渣。