Small bands of enemies still roam the Pelennor Fields, threatening all who venture forth.
'Even now, enemies still roam the Pelennor. If it is to be safe again for the Gondorian farmers who tend the land, we must defeat these remaining foes. I want the Pelennor safe before we march eastward.'
You should defeat any enemies you find on the after-battle Pelennor.
You should talk to Éomer, who can be found at Aragorn's Pavilion on the after-battle Pelennor.