Japanese maple not doing well

Answer 1

The container that it has been planted in is way too big. You have to stage these up slowly, unless you can gauge exactly how much water that it uses.

This has been over watered, or left in wet soil for too long. What happens when there are no roots in so much soil yet is that the water remains in the out regions of soil. This leads to stagnant soil and infection, which is what is going on here.

This is going to be a delicate balance of killing the infection without killing the tree.

First, you will need to put this in a container that is only a few inches bigger than the root ball, which will be roughly twice the original rootball size. It helps to have it in something where you can see roots coming out of the bottom, because this will need to pot it up once this happens, until it reaches the size necessary to put it into that large container. This may take a year or two.

The next step after going back to a smaller pot with fresh soil will be to treat the infection by applying a small amount of dolomitic lime (a teaspoon will be fine with a small container) and wettable sulfur (just a light dusting over the soil surface, to where you can barely see the dusting). This will take care of the infection.

From here, this article will help you to care for this until it is large enough to put into a large container:https://www.gardeningknowhow.com/ornamental/trees/japanese-maple/japanese-maples-in-containers.htm