Could be Liverwort. It grows on soil which isn't well-drained . . . I get it on my bonsai pots. But I could be wrong - can you put up a photo for a more accurate diagnosis?

Need a photo really, but if its not there all the time, but only when its wet and disappears again, likely a slime mould. Might be algae, but if its there all the time, then liverwort, maybe moss