the moss is growing there because your soil is acidic, shaded, compacted and somewhat moist. You will need to correct those problems before grass seed will grow. Putting topsoil down won't help much. And, growing anything under a maple is tough since their roots are shallow and voracious. Have you considered a moss garden? They are beautiful, restful, and low maintenance in the right place. I have included a link but a Google search will bring up a lot of info.

Here is a link that might be useful: moss gardening