This is a problem in some lawns, you have the type of grass pictured, that when you cut it, it appears to be very stringy and does not cut that well. Which means that it sticks out from the rest of your lawn and does not look good. This stringy grass is a landscape plant that is now a weed in your lawn.
This stringy grass is fountain grass. The same fountain grass that is found in a landscape setting. Bummer. What happens is the seeds blow into the lawn and start to germinate. Now you have fountain grass growing in your lawn, and it stands out because it does not cut well when the lawn is mowed.
The best way to deal with fountain grass is to treat it with the same product that you treat crabgrass plants with, quinclorac. It typically needs two applications about three to four weeks apart. But this will suppress the fountain grass is your lawn. Just remember, always follow all of the directions on the label when using any pesticide, including quinclorac.
The stringy grass does look unsightly, but there is hope. Patience will be needed as it will take a few weeks for the process, and then a few more weeks to see everything through. However, at the end of the process, you will selectively have removed the fountain grass from your lawn.