Looks like Marchantia polymorpha. Common liverwort. http://en.wikipedia.org/wiki/Marchantia_polymorpha https://www.google.co.uk/search?q=M...A&biw=1407&bih=678&sei=iJUCUMzXBImLhQfUqrCCCA
These March across the tops of pots that have compacted soil surfaces. They need a film of water to get started, same as ferns and mosses.