mildew, mold, moldy, mildewed
霉 centers on fungal growth caused by dampness: mildew and mold, and the resulting state of being moldy or mildewed.
霉 combines 雨 (rain) with 每, suggesting dampness and growth. The character likely expresses the idea of mold or mildew appearing in humid conditions.
Rain 雨 falls on something that has been left too long (每 can suggest repetition), and mold grows. Picture a damp, forgotten corner covered in mildew.
For バイ, imagine buying a loaf of bread and finding it covered in mold: buy -> バイ, and the moldy bread is a disappointment.