Q.Newly Potted; Just The 18” Trunk. Milky Substance Coming Out Of The Top.
Observed the substance about 30 minutes ago. Went to take a picture–just now–and it is gone. One might assume because the sun is on it??? Anyway, my neighbor brought it back from Hawaii and I would feel horrible if I killed it.

Certified GKH Gardening Expert
I'm guessing it is a plumeria stick that you are rooting (or it may have been rooted already). Plumerias have a milky sap so don't be alarmed. These articles should help:
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria-cuttings.htm
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria-plants-indoors.htm
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/how-to-fertilize-plumeria.htm