I concur; aphids. And about as bad an infestation as they come.
They're not hardy at all, so most things will kill them easily enough. It's keeping them away that's the problem. Miss just a couple, or miss their eggs and they come right back.
I've heard people recommend setting the whole tree completely submerged in a bucket of water overnight. Drowns them, and little chance of missing any. Does nothing for the eggs, so you'd have to do it again a few days later, and again after that. This gets the later generations.
I've never actually tried it myself, but I've yet to see the flaw in the logic. Just a matter of whether the tree will manage being submerged for 4 to 8 hours at a time every few days for a couple weeks.