try googling liverwort its a type of algae usually seen in damp areas especially in plantpots-- I know yours in on the grass but the description reminded me of this

Probably Dog Lichen if its in the lawn.

http://www.dgsgardening.btinternet.co.uk/lichen.htm

Its either a lichen or algae - if it starts out black but curls at the edges and goes pale when its dry, its lichen. Either way, both these are a sign of a poorly kept lawn - improve your lawn care routines, making sure you scarify and aerate in autumn, apply something like Autumn Toplawn or a lawn feed which is suitable for autumn use in September and make sure you feed it with something like Lawn weed and feed, or just lawn feed, in mid April and then again end of May next year. Mosskiller should get rid of the problem right now, but I'd wait till autumn and apply a moss treatment at the same time as the autumn feed.