Wow. That is interesting. I'm new to hoyas so it beats me but I am anxious to see what a more experienced grower has to say. Good luck with it.

I thought it might be part of the plant growth too, but I don't believe it is roots. You can see in this photo that it has grown a nice healthy group of roots. I did go ahead and scrape off the substance, though. The new shoot at the top was not producing any leaves, and I was worried the stuff might be affecting the plant. Still wonder what it was though. Thanks for your thoughts.

Did it scape off easy? Weird.....i was thinking roots too since a cutting i have in soil grew knubby roots because i accidentally planted it upside down.

HaHa! Oops. Amazing how determined the hoya is to survive and thrive.

The bits I removed from my Hoya stem were somewhat hard and did come off fairly easily. Thanks for your thoughts on this issue. I'll continue to monitor the plant to see if the "stuff" returns.

Could it be mineral deposits from your water?

It looks like its/was callus, basically a blob of undifferentiated cells that grow around wounds, for example on the end of a cutting. A normal part of the rooting process.

Heres a willow:

But also quite typical when you root Brugmansia:

or Hibiscus:

Any time I root a hoya in water, once the roots get between 1/2-1”, I move the plant to soil. That way there isn’t a huge adjustment for the roots. I don’t think your plant likes being in that vase. I would move it to soil before you have to damage roots to remove it from that container.

My cane begonia cuttings started in water often develop the same thing. It has not negated rooting, but I have wondered if it was a good thing or a bad thing.