Your single leaf Sweetheart Hoya (Hoya kerrii) must have had a part of the main stem attached because that sure looks to be either a new stem sprouting, or possibly a peduncle (bloom spur)! I'm having eyesight issues today but it sure looks like a bloom spur with tiny buds at the tip.
For comparison, these are photos of different Hoya's showing peduncles with tiny buds:
