It's scale. You need a multiple approach.
Scale secretes a material which attracts ants, and ants help "move" the scale around. Also the residue can cause a black mould. So you need to check for, and treat, ants.
Hard-shelled scale are difficult to kill, but sometimes what you are looking at are already deceased adult scale, and they have already moved on to healthier plant material. Young scale are harder to detect, much smaller and usually green or yellowish. Young scale can be treated with chemicals which blanket and smother them (a few treatments are usually needed to resolve the issue with newly hatched scale). A few strong bursts from the hose will then usually dislodge the dead scale.
Older scale (such as you photographed) can sometimes be removed by gently scraping off).
What chemicals or treatment you chose to use depends on many factors - some are very strong and dangerous, others slightly less so but can be made up at home (soapy ones) - any may impact on other plants, beneficial insects, edible crops grown nearby, pets, children or native insects/wildlife.
To be honest, your sheena's lower dead growth may never recover whatever you do to the top, if it were me I'd be inclined to give it a really BIG haircut- like as hard as you dare go - and dispose of all the clippings, give it a good feed and hope for lots of new (pest-free) re-growth. You could take a few cuttings to propagate just in case, and just cope with any lack of privacy issues etc in the meantime