Lichen (Candelariella aurella)
Here is a common site on brick buildings and walls as well as on roofing tiles and even roofing felt, Candelariella aurella. This is a mustard coloured lichen that grows outwards in a oval shape, gradually expanding in size as it grows. Once it gets established those dish-shaped discs appear which are the fruiting bodies from which the tiniest of spores are released.
It does no harm to its substrate (or the building in general) but it is tempting to clean it off which is a shame as it is easy to forget it is a living organism that takes years to grow to any size. In any event, it is so successful in its environment you will soon have some more to replace it.
It is very common in the south (and so it is in Dorset) and east of England as it is relatively polution tolerant. There are a couple of other similar species but they are less common on brickwork.
Find out more about the nature of Dorset at my website www.natureofdorset.co.uk