I am not surprised that your tree is diseased. of all the maples, acer
is the most prone. it is also the fastest to decay. what you
described is probably a fungus that affects the stems. if you want to
try to save the tree, start pruning. start at the tips of the major
limbs and work your way down til you see healthy wood. then treat the
tree and the area with a good copper fungicide. not much hope.