Holding shift whilst placing the water tile near the edges of the bridge will get rid of the grass tiles. It's a good idea to get acquainted with this technique as it leads to awesomeness. Just an FYI.
Thanks, it really does work. I never knew that trick existed, man that's a time saver. When I went in and fixed it, I had to change the bridge pieces to Upper Level pieces, which took way more time than if I had used this technique.