Moulds are a large and taxonomically diverse number of fungal species in which the growth of hyphae results in discolouration in walls and paintwork.

Lifecycle

The network of tubular branching hyphae, called a mycelium, is considered a single organism. The hyphae are generally transparent, so the mycelium appears like very fine, fluffy white threads over the surface. Cross-walls (septa) may delimit connected compartments along the hyphae, each containing one or multiple, genetically identical nuclei. The dusty texture of many moulds is caused by profuse production of asexual spores (conidia) formed by differentiation at the ends of hyphae. 

Damage

Moulds cause biodegradation of natural materials, which can cause damage to property.  It occurs when there is an excess of moisture in the air, which reacts with surfaces that have a different temperature. The excessive moisture can be caused in many ways, for example from condensation, leaking pipework, rising damp, faulty roofs or damage to external walls.