What's this weed and how do I eradicate it???
This is chickweed. It is a winter annual, meaning that it germinates in fall, lives through the winter, and has already flowered, is going to seed and will die soon and disappear.
At this point there is no treatment. You can pull what's left if it doesn't pull your grass up too.
Here is our lawn weed gallery with photos and more information.
Chickweed is the first entry: http://extension.umd.edu/hgic/lawns/lawn-weed-identification