It's a liverwort - indicates soil which is damp most of the time and hasn't been turned over at all. Turning it over in a pot isn't usually an option, but I note you have trays beneath your pots which means the plants are sitting in water. Remove the trays and allow the pots to drain properly, take a small hand fork and disturb the soil on top of the pot to remove the liverworts. The only time trays under pots are appropriate outdoors is if the weather is very hot and dry and potted plants are drying out too quickly, or have dried out so much the water just runs straight through without penetrating the soil.

Ooh lovely, Marchantia polymorpha!

my thoughts exactly landgirl.
the advice given by bamboo is spot on.
liverworts are a relatively simple plants but with a fascinating lifecycle. an early plant in the evolution of the plant kingdom.