thank you marckGreetings, welcome to the Forums.
This looks like a rooted leaf cutting of Variegated Heart-leaf Hoya (Hoya kerrii 'Variegata') that is beginning to rot. So yes, it is a mold or similar fungus.
By the way, a rooted Hoya Leaf will not develop into an entire plant. It will only remain a leaf and will eventually die as it cannot grow further. Such leaves of Hoya kerrii are sometimes sold as novelties around St. Valentine's Day.
If you want to propagate a Hoya vine you must use a stem cutting.
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