I can't tell if the aluminum bang plate has shifted up or if the outer edge has just corroded away. This may not be serious but does indicate that moisture has or will get under the bang plate. That moisture may or may not soak into the transom.
If you tap the bang plate, can you see it move up and down? If so, you need to decide if you want to open things up and that will mean lifting the motor. If it seems solid, for now you can sand/grind away the corroded edge to clean it up and seal with a good caulk like LifeSeal.
Down the road, a moisture test will document if and and how far any water has penetrated the transom coring.
It is a good sign that the motor mounting bolts are not oozing rust stains. That tells me that the transom, at least behind those bolts, is probably dry.