Answer: There are two insect pests that fit your description - Cottony Cushion Scale and Mealybugs. Both pests feed on the fluids within leaf and stem tissues. Mealybugs and Scale insects excrete a sticky substance that eventually turns black (Sooty Mold). Scales have natural predators (a wasp and a beetle) so populations are usually kept in check without resorting to insecticides. Mealybugs are favorite meals of ladybugs, lacewings and syrphid flies.
If the infestation isn't too widespread, you can simply remove the leaves with populations of the pests. Don't remove move than one-third of the leaves, though, or you'll stress your hydrangeas too much. If you feel you need to treat the pests with a spray, use Insecticidal Soap against them.
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