Here is the response my husband got from a coworker that is a plant pathologist formerly from the University of Fl.

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You created perfect conditions! This looks like Meliola, which is a really cool fungus that is often called a black mold. There are a couple species reported on Hoya, but not in the US. You may have a first report!

I have never worked with Meliola spp., but suspect Compass or Trigo would control it. These molds will usually go away with less favorable conditions. I'm curious to see what happens.

Thanks for sharing,
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Subject: FW: Hoya INCRASSATA

I hope you are doing well. I have a diagnostic question for you. John asked to be copied as he is curious what your opinion is.

The picture is the leaf of a hoya incrassate. My wife has hoyas hanging in front of windows in our kitchen (faces north) which gets good sun in the late morning. There is a ceiling fan for air movement (when it's on!). We went away this past weekend and the hoya was fine on Friday, but by Sunday afternoon when we returned, the black mold appeared. We had the ceiling fan off, so literally no air movement, and it was fairly warm outside (~80), so they got good sun. I'm pretty sure it's not aphids. No visible insects and these plants get a fair share of imidacloprid and spirotetramat. My wife tried washing the leaves, but the mold wouldn't come off.

Thanks for your help.

Pete