Might these be Nepeta seedlings?
These might be too small to ID, but I found the ground between a daylily, a mum and my Nepeta 'Blue Wonder' covered with these guys. There are lots more of them on the others sides of the Nepeta too. The odd thing is 'Blue Wonder' is suppose to be sterile.
I know they aren't daylily seedlings, I don't think they are mum seedlings, so could they be from the Nepeta? Are sterile plants sometimes not so sterile?
I really didn't have any weed problems last year, so I doubt it's some infestation from a weed.