A mold (. ) or mould (. ) is a fungus that grows in the form of multicellular filaments called hyphae. In contrast, fungi that can adopt a single celled growth habit are called yeasts.Molds are a large and taxonomically diverse number of fungal species where the growth of hyphae results in discoloration and a fuzzy appearance, especially on food.n contrast, fungi that can adopt a single celled growth habit are called yeasts. Molds are a large and taxonomically diverse number of fungal species where the growth of hyphae results in discoloration and a fuzzy appearance, especially on food.