What kind of weed is this
Japanese stiltgrass is a foreign invasive plant that is wreaking ecological havoc all over the Eastern United States. Research is ongoing. We were swamped with questions about Japanese stiltgrass last year, probably because weather conditions were optimal for it and it is increasing exponentially all the time.
Pull it out of your beds before it goes to seed in late summer/early fall. (Keep an eye on the tips of the blades, where you will see the seedheads emerge.) If you can't pull it all, spray or cut down the plants before the seeds mature.
In the lawn, use a crabgrass pre-emergent herbicide in early spring (early to mid-March). Since it germinates before crabgrass the pre-emergent should be applied a couple of weeks before you would apply it to control crabgrass. Here is our webpage (be sure to read the links, too) http://extension.umd.edu/hgic/problems/japanese-stiltgrass
Seeds of Japanese stiltgrass stay viable in the soil for up to 5 years, so one big key is to keep seeds out of the soil. If you have a mower that can bag clippings, use it when the stiltgrass is going to seed late August through early September. The rest of the year, we don't recommend bagging clippings. Also, be sure that the mower does not shoot clippings into beds or natural areas.
Rutgers University has more comprehensive information on control options in landscape beds and lawns. https://njaes.rutgers.edu/fs1237/