For some reason, Houzz sometimes doesn't allow a picture in the original top post. But it will in a comment. Post your pic in a comment back to us.

Caroline, is your hoya outdoors? Those look like whiteflies.

OP says the plant is in a conservatory. They could be greenhouse white fly. Or possibly cast aphid skins. If you put some on a piece of paper do they move? Could you look at them though a lens?

I will have a closer look