Hm.. I would have bet on spider mites. But the soap treatment should have worked. Have tried tapping the branches over a sheet of paper? You would normally get tiny dot on the paper, which you can smear out in tiny brown/red lines. If you do, you have spider mites. Treat accordingly.
Else... Is it very humid where you live? Some form of algea / lichen or fungal ?
It looks like the growing tips are healthy?