I do not know what a dust cloud is but it is possible what you are seeing are tiny white flies or fungus gnats arising from the soil when you water. If the plant looks healthy then the fungus gnats are most likely feeding on organic matter in the soil and not harming the plant.  I suggest you get a magnifying glass and check the back of the leaves for tiny white flies. If you see them or the eggs on the heuchera you can try spraying a strong jet of water on the leaves at least twice a day for a week. This disturbs the colony so the flies may leave. Otherwise when a perennial is so infested with insects that a cloud arises you should decide how healthy the plant looks. If is not wilting or disfigured it is up to you to keep it our toss it. Some insect invasions are seasonal but often they will destroy a plant as summer arrives. Good luck and Keep growing, Marianne Binetti