Are you sure it's mold & not mealy bugs?

I am also surprised by this diagnosis. I'm not sure that I've ever seen mold on a Hoya, aside from the plant rotting from the roots upward.

Also, once the plants in one of my aquariums developed this black stuff that I cured with a lemon juice concoction.

It is definitely mold. It's possible it's been happening from the roots up. Essentially, the mold has covered the leaves/stems and the leaves yellow and eventually fall off.

I have one solid piece with established roots. I just want to eradicate all mold before I transplant it.

Is it possible you have sooty mold caused by a pest like aphids? In that case, once you kill the bugs, you can just wash off the mold and it won't reappear.

Neem oil will kill most such insects, or you can juice a lemon and mix it with water and use that as a spray. I did not find that any of the Hoyas I treated with this concoction reacted badly. I also cleaned the leaves off with soapy water. For me this eradicated whatever problem I had. I'm not sure that you're dealing with the same thing.

If it's a fungal infection you're dealing with, cinnamon would be the best bet. You can make a paste out of it with water and apply to the affected area. We also have some members here that swear by using an athlete's foot spray.