Invasive weed in lawn
We think this looks like an invasive weed called Japanese stiltgrass.
It is a summer annual that is just getting started. It should be easy to pull. Be sure to not let it flower and set seed.
It germinates a little earlier than crabgrass, so a pre-emergent, which stops seed germination is the long-term goal for next year.
Here is our weed gallery page about it: http://extension.umd.edu/hgic/japanese-stiltgrass
Chickweed's life cycle is different. It is a winter annual, meaning that it germinates in the fall, exists in the winter, but then really gets going blooming and setting seed in the early spring. Right now, it is done and getting ready to die out for the year. Here is the page on chickweed and it's management: