Algae and algae-like growths: A green film or powdery deposit is typical of algae on paving, stonework and garden furniture. The dark green or blackish jelly-like growths that often appear in damper, cooler weather on paths and areas of tarmac are incorrectly known as blue-green or gelatinous algae, but are in fact a cyanobacteria called Nostoc.
Lichen: These are common on paving and timber structures such as garden benches. The colour of lichen varies with species, but most are silver-grey, grey-green, yellow or orange. They can be crust-like, leafy or scurfy in texture.
Liverwort: Liverworts that grow on hard surfaces usually have a green, flattened, plate-like body and no leaves. A common example is Marchantia, which is often topped with umbrella-like structures carrying sexual organs.
Moss: Mosses commonly found on hard surfaces are usually cushion-like.