Yes these are Periphyllus aphids a group of aphids that are common on maples, and it is probably P. californiensis, as they are most common on Japanese maple. The small white dots are nymphs. I see also some older nymphs that are a bit more leggy. Those are not gnats but the winged form of the aphid, or alates. The cluster of yellow eggs are lady beetle eggs, and the larvae that will hatch from those eggs are voracious predators of aphids.
I like your management approach with high pressure water, this can be effective to disrupt the aphids and reduce their populations. I suspect the cool spring weather was favorable for the aphids. One consolation is that now that it has warmed up some, the natural enemies of the aphids will be more active against them. Usually there is lag time between a pest population cycle and the response of natural enemies. Here you can see the lady beetle larvae are moving in and probably will adequately control the aphids along with other predators and parasitoids wasps.

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