Help, Please! Sick Braided Tropical Hibiscus

Hello - I am new to Dave's. I have two braided TROPICALS which I've had for a year. I have another I've had for 4 years which is thriving, so I have a little bit of experience. One of the new ones was looking tired after overwintering indoors - about a month ago - and upon inspection I noticed that one of the 4 braided stems was looking shriveled and was soft. Around the stem toward the top were hundreds of white specks, like tiny straight lines, stuck to the stem, but nowhere else on the plant. A local plant expert said she thought it was aphid casts, but the plant was indoors and I never saw a single insect on it. Only that one of the 4 braided stems seems to be affected. I think - dread - that it is actually dead now - no new growth or leaves in sight.
Any ideas what has happened to my cherished little tree? And should I cut the affected stem away, or can I leave it in the braid? I sprayed the plant right away with soapy water w/ trace of alcohol in it and changed the top few inches of soil on the recommendation of the woman I consulted. Didn't seem to change anything. Now the plant looks a bit withered - all leaves are drooping as if it needs water, but the other 3 stems have miniscule signs of growth.
Does this sound familiar to anyone? From what I've read, it doesn't seem to fit the aphid descriptions...also, the white casts were only on the stem, not any leaves. A few leaves have black spots about 1/4 inch around, but these don't seem to be related and there are only a handful.
I would truly appreciate any feedback you may have, in hopes of saving the tree. It bloomed profusely last year with 4 different colors, and is really special to me! Many thanks for any help offered.

Sounds like the white scale we have on some of ours, can be almost non existant, then bam they explode and multiply quickly.Can kill branches if they get real thick on the stems. see pic. these were on the plants when they were shipped from the growers in Fl.