一粒老鼠屎坏了一锅粥/ 一粒老鼠屎壞了一鍋粥yīlìlǎoshǔshǐhuàileyīguōzhōuone bad apple spoils the bunch
Literally means one pellet of rat droppings ruined a whole pot of congee; used to describe how one small negative element or person can ruin something otherwise good.