一颗老鼠屎坏了一锅粥/ 一顆老鼠屎壞了一鍋粥yīkēlǎoshǔshǐhuàileyīguōzhōuone bad apple spoils the bunch
Literally "one pellet of rat feces spoiled the whole pot of congee"; used to describe how a single small negative element can ruin an entire situation or group.