Liverworts are fascinating! They're descendants of some of the first plants to colonise the land. Your liverwort is Marchantia polymorpha, which is really common in pots and, if you look closely, you'll see some cup-like structures on the surface. These contain tiny propagules called 'gemmae', which are dispersed by rain drops, spreading the liverwort far and wide! Liverworts don't harm other plants, but they're a symptom of excessive moisture, so as long as you keep your compost on the dry side, you shouldn't have to worry about them.