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The pest on the String of Hearts (Ceropegia woodii) in the OP are a species of Scale Insect (Coccoidea, Hemiptera).
Remove any visible scale with a tool (a sharpened pencil works well), and then spray the plant with insecticidal soap. Repeat both steps as necessary until the infestation has abated. One may prune off the worst-affected stems to make treatment easier and more effective. Starting over by striking clean cuttings in fresh soil is also an excellent option.
String of Hearts (Ceropegia woodii), also known as Rosary Vine, is a succulent trailing plant in the Dogbane Family (Apocynaceae), native to southeastern Africa. It is commonly grown by succulent fanciers and can make a fine houseplant in colder climates.