Aphids drip honeydew excrement, which leaves behind white/sticky coating on leaves below.
I refuse to use chemical controls for aphids. A heavy spray from a hose end attachment...
...usually does the trick and unfortunately, the colony was way too thick. I would waste too much water if used alone.
As I sprayed, I squished nearly all the soaked stems by hand. Yes, gross, but easy enough. Very cathartic to rid them in this way.
All clean. Easily said than done. I will have to keep my eyes on these stems. Colonies can easily regroup and multiply.
Hopefully, the left side will now have enough energy and sap flow to regain what was lost from the aphids feeding.
The likely cause: very dry May weather. This site is usually quite damp and moist. Little rain and no irrigation have made aphids attracted to this succulent feast.