You may be seeing unsightly plants in your tall fescue lawn similar to what is in the photo. This is probably annual ryegrass or orchardgrass. Both can come with seed put out in the fall or with straw if the lawn was mulched. Annual ryegrass is a common weed in wheat production.
There is no selective control for either of these problem grasses.
Annual ryegrass will most likely die out as temperatures get hot. Orchardgrass will weaken but possibly not go completely away.
You can control either with glyphosate (Roundup) applied directly to the plant, but some of the surrounding turf may also be killed. To keep these from spreading, make certain you mow often enough to keep them from forming seedheads and setting seed which may germinate this fall.
Drs. Charles Peacock, Fred Yelverton and Travis Gannon
Turf and Weed Management Specialists, Dept. of Crop Science, NC State Univ.