かび (～がはえる)
かび (～がはえる) · kabi (～gahaeru) · JLPT N2 · noun
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🦠 黴 = black (黒) + tiny (微): mold is tiny black fungus spreading on food
The verb phrase かびがはえる means mold grows, since はえる means sprout or grow. The kanji 黴 combines 黒 (black) and 微 (tiny), matching the tiny black spots of mold.