I've seen tons of soccer games being played on the mall. If you are truly just talking about a game of catch, I don't see how it would be a problem if you were there, too (assuming you really mean on the mall and not at the softball fields near the Washington Monument). That said, I've seen those soccer games start at noon (but just a couple so they aren't taking up the entire length). The later you get there, the more competion you have to find space.