Algae requires moisture to grow and reproduce so it thrives in damp, sheltered or shaded areas. Growth tends to be more significant in autumn and winter, but can happen at any time if the surface is persistently damp and shady. Algae can be different colours - such as red, green and black - depending on the environment that it grows in. Algae near the seaside often appears red in colour due to marine salts in the atmosphere.
In the built environment algae typically affects hard exterior surfaces such as stone or brick façades or block paving, It creates unsightly stains and can contribute to the degeneration of the surface material. It can also make exterior terraces, pathways and steps slippery and potentially hazardous.