mold, mildew
黴 centers on fungal growth that appears on damp surfaces: mold and mildew. Both meanings refer to the same fuzzy, spreading organism.
黴 is a phono-semantic compound. The bottom 黑 (black) suggests the dark color of mold, while the upper part 微 (minus 彳) likely contributes the sound. The exact historical development of the upper structure is uncertain.
The left side 彳 suggests creeping movement, and the bottom 黑 shows the dark color. Picture mold slowly creeping across a surface, leaving blackish spots.
For バイ, imagine buying a loaf of bread only to find it covered in mold: buy -> バイ, and the moldy bread is a disappointment.