Q.I Have Japenese Maple With A Trunk Problem
Please see photo of trunk problem on my acer. The leaves are fine
Certified GKH Gardening Expert
That is a severe scale insect infestation! You will need to remove all that you can, manually. They seal themselves to your plants, so chemical treatment will do nothing until they are scraped.
Once you scrape and treat, keep an eye out. The eggs are still around, and likely, some will get missed. Keep on them with this treatments for a week or two, and they will be controlled.
Here are some articles that will help: