My guess is quack grass. It's hard to get rid off. I've usually resorted to pulling it out by hand. I think any herbicide that kills it would also damage your lawn, but I might be wrong.

Also looks like common bermuda. Where do you live? It's important.

Whatever it is, it's a grassy plant. Killing unwanted grass plants among other grass plants is very hard to do. If it is just now getting started, I would suggest biting the bullet and using something like RoundUp on it NOW before it spreads further. You will have to either reseed with KBG or be very patient as the KBG refills into the sprayed out part.