It's a liver wort :o) It likes to grow in damp places.

it's harmless and a primitive plant, present even before the dinosaurs.. just scrape it off.

It may be 3cm across now but in a few weeks it will have multipled by a large amount. You should tweek it out now or it will smother any plants and prevent overhead water getting to the compost. it is probabe that the soil is poorly drained and too wet at the moment.
This is not a critisism of you, Gg, I have a lot of problem with liverwort, myself.

Have a look at my photos - the fruiting body of that little plant. Like a little alien

It looks as if its watching you doesn't it?

Thank you all, it is a weird looking thing close up!