Looks like oxalis. The distinct heart shaped leaves are the giveaway. Mine is all pretty well grown now and bigger than everything else so it's a lot easier for me to tell on sight. It will get easier for you to tell as you continue to see it. It is also a darker, more solid green than the clover that grows in my yard, and it sometimes gets black speckles on it.
It also has several stems that gather down into one root. I try to yank it up now as I come across it. It pulls up pretty easy as it is all attached to that one root.