Answer: The substance on your butterfly bush might be cottony scale, mealybugs (cottony fluff) or it might be lichen (a fungal type growth). Mealybugs and scale insects can be damaging so controlling them is important. Lichen is just unattractive but it won't hurt your plants.
To get your bushes to fill out, do some pruning. Try to keep the top more slender than the bottom. If the top gets too large it will shade the bottom of the shrub and no new leaves will grow. So just trim it back all over to encourage it to fill out and try to make the top more narrow than the bottom.
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