There are a number of fungal issues or blights that can affect Japanese maples, most of which are rather serious problems that can often mean failure/death of the tree. With sharp pruners or pruning saw cut off the affected branch as close to the main trunk as the branch collar and take this portion into your local cooperative extension office for an accurate diagnosis. Sterilize your pruning tools thoroughly after making this cut.

If it is one of these issues (and from the look, I would lean heavily to verticillium wilt), there is little you can do other than keep the tree in the best possible condition. IOW, there is no magic elixir that will "cure" the problem. Sometimes otherwise healthy trees can outgrow these problems, most often it is just the beginning of the end, unfortunately. Sometimes the progressive decline is very gradual; othertimes it can be extremely rapid.