A picture would really be helpful. We can't be certain of whtat you have without it, and it is difficult to be sure without seeing the plant in person.

However, a common insect that resides on some of the stems of my japanese maples is the wooly aphid. The white fluffy insects feed off of the sap in the young stems by piercing the thin bark. They eventually form a fluffy white "bed" around them which makes it difficult sometimes to see that there is an insect there. Also, if you get near them they often jump away leaving only the fluffy "bed". There are probably many other things it could be but this seems to be the most common in my area. These insects usually don't do much harm to the tree unless there are a large number of them. Insecticides are not recommended for jap. maples, and it would have to be covered with them to be in any danger in my opinion. If so, an insectide soap or oil that wouldn't harm the tree should be used but only during the cool temperatures during early morning or evening to prevent leaf scorch.

Powdery mildew can grow on the leaves of japanese maples among other plants, but I've never seen anything on stems. It is also usually not dangerous to the plant, just unsightly. Most insecticides should take care of this fungus if it was a must.

The wooly adelgid is the horible insect that is wiping out the hemlock population on the east coast, but I've never seen it attack a maple.

thanks for the imput, if i can figure out how to get a picture on here, i can do that...i can take one tomorrow when i get out of work.