It's commonly known as dog lichen, and it affects poorly drained, compacted lawns. It's actually a cyanobacteria, and where this is, you usually find other lichens, liverworts and mosses. You can use chemical treatments, but it will always return unless you address the underlying cause. Its basically a symptom or sign that your lawn is seriously in need of some care and attention, so things like aerating it (spiking with a fork all over, application of some sharp sand, feeding during summer and so on) - details in the link below

https://www.rhs.org.uk/advice/profile?pid=414

yes I fully agree with Bamboo. we had a small area of it and OH tackled it with the fork and some sand to brush into the holes made by the fork. If I remember he did use the rhs website to sort it.