plesan

1. mold; mildew
   Na hlebu se pojavila plesan, moramo ga baciti. — Mold has appeared on the bread, we have to throw it out.
   Zbog vlage u kupatilu ima plesni na zidu. — Because of the damp in the bathroom, there's mold on the wall.

In everyday speech in Serbia buđ is more common; the adjective is plesniv.