Yes, probably over-watered in soil that holds too much water.
But that doesn't account for the white specks. Hard water can leave a white crust behind on the soil, but that would be just a flat deposit and more likely on the soil than the tree. It wouldn't be so three dimensional.
I expect your diagnosis of aphids, maybe woolly aphids, or something like that, is accurate. Spray your tree with insecticidal soap or Neem oil to get rid of them.
But you waited a long time to come and ask. Your tree is in very bad shape and may not be salvageable.
Before you replace it, read the articles here
https://www.helpfulgardener.com/bonsai/ about bonsai care, bonsai soil, watering ,etc.
Here's a thread about bonsai soils :
https://www.helpfulgardener.com/forum/vi ... =36&t=3422 with links to more info.