No, treatment is not needed, nor would it likely be effective even if it was warranted. Given their size and color, what you're seeing is probably springtails, in which case they are a natural component of soil and not damaging to plants. They feed on organic matter and thrive on moisture, so if abundant in a given area, you might have had some rotting leaves in that area, lots of compost or manure applied to the soil, or maybe already-dead roots that they were feeding on as the roots disintegrated from decay. (The linked page is very basic, and springtails come in an array of colors beyond the tan species pictured, including white and silvery.)

It's hard to guess as the cause of the plant's symptoms without photos, but it could have been under-watering, over-watering (symptoms of the two can look very similar), or a fungal infection as the typical culprits. Given the presence of springtails, perhaps over-watering was a factor, unless there was a leaking or dripping hose spigot in the area, or nearby roof downspout outlet. Morning sun should be fine for any hydrangea species, although some prefer or tolerate more all-day sun than others.

Miri