There's not much you can do in this situation. Fungicides are available, but are dangerous as well. If your maple is large enough, you can cut away the affected area, and then a couple inches extra as well.
Cut back to clean wood, sterilizing knive between cuts. When dormant spray with fungicide, available at any home supply store. Also strengthen tree with low nitrogen fertilizer, one labled with zero as the first number..
If you have a can of regular "non oil based" paint already in your home, take a paint brush and brush the areas that have fungus growing on it. If it is a large tree just make sure that you get the area closest to the trunk.
To get rid of the fungus use an approved fungicide specific to Japanese Red Maples. A well drained area is important for the health of your tree. Push back any mulch and use care when mowing or trimming not to strike the base of the tree. Cover any pruning cuts with a sealer.
