Are you able to share larger image files? Unfortunately these are too small (less than 1MB) for us to see enough detail and enlarge them. Did you apply any topsoil or compost top-dressing to that bed before the mystery plants germinated? If so, it might have been contaminated with weed seeds, unless a nearby wild plant went to seed around that time and they landed in this area and germinated. This does not look like a typical farm cover crop. Until we can identify it, it probably won't hurt to keep it as if it were an intentional cover crop, removing it when you're ready to plant again. Keep it from producing seed after flowering, though, to prevent more seeds from infiltrating the soil. We can't tell what flowers will look like, though, or when they'll appear, until we can get a better sense of its ID. If you're able to send larger images or take more close-up photos showing leaf arrangement and other traits (does it have a scent when bruised?), we can try to narrow-down what it might be.

Miri