It grows on surfaces in masses of branching threads which resemble dense cobwebs. Spores are carried by air currents or by adhering to insects or animals.
Active mould can be any colour, depending on the species and what it is growing on.
Mildew is another type of fungus, similar in structure to mould, but distinct as one species of fungus is distinct from another, such as yeasts, rusts, and mushrooms. Unlike plants, which produce their own food, fungi absorb nutrients from dead or living organic matter. There are over 100,000 known living species of fungus and mycologists estimate that there may be as many as 200,000 more unidentified species. The spores of fungi that become mould or mildew are always present in the air and on objects. Basically, mould and mildew eat library materials.