Yes, there is a very effective treatment for these types of vegetable growths, which are most likely to occur in damp conditions or on surfaces with a high moisture content. Before treating, first repair the source of any leaks and check for blocked air bricks and defective or missing DPC (damp proof course). Allow to thoroughly dry then treat affected areas with a good fungicidal wash. Leave for 24 hours, wash down to remove residues and allow to dry thoroughly before applying coating. Isolated algae patches on a building exterior may indicate structural defects and should be investigated by a reputable builder and/or building surveyor.