Certified GKH Gardening Expert
This is common Liverwort. It is a non-vascular plant, so herbicides may not be effective. Boiling water will kill them off. Pour boiling water 2 or 3 times per day for a week or so, and it will get rid of them. Your soil will be perfectly clean for planting, immediately, after treatment, unlike with herbicides!
Here is an article that will explain a little more about these plants: https://www.gardeningknowhow.com/ornamental/water-plants/liverworts/liverwort-information.htm