The modern “Turf Type” tall fescue that we plant today is descended from Old Fashioned Kentucky 31 Type Tall Fescue. These patches will tend to stay green when the rest of the lawn turns brown for the summer. Many times, Old Fashioned Tall Fescue is mistaken for Crabgrass, which it is not. It’s not a big problem at all if it doesn’t bother you.
Super Seed with Modern Turf Type Tall Fescue. This new blend a little better with the Old Fashioned Tall Fescue. Also, consider killing the areas or the lawn, (if it is widespread) then reseed the lawn (renovation).