This is a horribly invasive non-native weed known as Japanese stiltgrass. It is very aggressive and a major destructive force in our parks and natural areas, out-competing the native plants that our natural wildlife need to survive.

It's also a terrible lawn nuisance. Here is our webpage on how to tackle it: https://extension.umd.edu/hgic/topics/japanese-stiltgrass  You will have to treat your lawn yearly, but if your lawn is small enough and it is just appearing, you may be able to nip it in the bud by pulling this summer before it seeds.  Be sure to get all the stems that have rooted. 

Ellen