This looks like nimblewill, which is a perennial grassy weed. It is challenging to control.
Here is our page on it: http://www.extension.umd.edu/hgic/nimblewill
Given that it is so interspersed with your grass, you may consider chemical control, as opposed to digging it (mechanical control). If you have more weeds than grass in this area, we'd suggest a lawn renovation by spraying it dead in late summer and re-seeding the area.
Another option is the use of mesotrione, which is the active ingredient of the product called Tenacity. It can be purchased on-line. It is not inexpensive, but you use a very small amount at a time and it works well.