It’s a super-cool groundcover called brass buttons (Leptinella squalida ‘Platt’s Black’). Hardy in Zones 4-10, I have it thriving in my garden on the east side of the house, where it’s protected from the afternoon sun. The plant grows about half an inch tall and creates a delightful spreading carpet of purple-and-green foliage that looks kind of ferny. When friends visit my garden they’re almost always fascinated; many say “it looks like little worms!”
Brass buttons is a low-care plant that does best in moist, well-drained soil rich in organic matter. It doesn’t like drought, and can be walked on a little bit without harming it too much.