These are aphids, most likely the Norway maple aphid. While they cause more damage in the spring, they return to accumulate at the bases of the trees in the fall. If this is the first time that you've seen them, then I would take a wait and see approach next spring. Some of the aphids that you're seeing now have laid eggs on the trees that will overwinter and give rise to next year's aphids, but none of the aphids on the tree now will survive the winter. If you see them every year or see an unusual number next spring then consider treating the tree with one of the insecticides suggested in http://extension.colostate.edu/topic-areas/insects/aphids-on-shade-trees-and-ornamentals-5-511/.