Mold was a type of multicellular fungus, related to mildew.
According to Deanna Troi in 2366, the Sacred Chalice of Rixx was "an old clay pot with mold growing inside it.". (TNG: "Ménage à Troi")
Molds could also be used to cure diseases; several molds native to Orellius were used to cure diseases by the crew of the SS Santa Maria. (DS9: "Paradise")