Nearly impossible to determine cause of poor plant health of this Japanese maple from photos. Reason for poor health may be cultural (soil, water, etc.) or plant may have a fungal disease (maybe Verticillium wilt) which is usually lethal. The white on the bark looks like fungal growth which is most likely a secondary infection due to the death/poor health of the tree. If tree dies, then do not plant another Japanese maple (in case of the fungal disease).