Not a moss but a liverwort. Yours appears to be the common liverwort, Marchantia polymorpha. It thrives under continually moist and fertile conditions. Sometimes it is considered a weed by nurseries because it thrives in pots that are kept under sprinklers and given regular doses of fertilizer.

Considered a pest & often comes with purchased plants that have spent extended time in a pot at a nursery. I believe I read it likes acidic soil & dampness, so removing it then raising the ph of your garden bed may prevent regrowth. It can spread fast in the right conditions. In western WA I see it a lot & promptly toss in the trash or the firepit for burning later.