Possibly mealybugs (related to scale) or scale. Anything you use against scale can also be used against the other. If the infestation is still growing, Imidacloprid could help but if the pests have left (take pictures and look for movement) then the insecticide may not do much since they "left the nest" already. I personally would apply it once any way this year. But then I would start reapplying it next year about 1-2 months before this time of the year. If you prefer a more organic solution, you could squish them by hand, release beneficial insects such as lady bugs (beneficial insects) or apply insecticidal soaps, Safer's Ultra Fine oil, dab them with alcohol, etc. instead of using Imidacloprid. Do not use the insecticide and the beneficial insects at the same time because the insecticide will kill the beneficial insects. Assume they will reappear next year and apply counter measures about a month before the problem was first detected.
