And what should I do about them? I'm surprised they are not attacking the nearby tree. How would these all of a sudden appear on this plant?
Power hose them off, then repeat as needed.
They appear to be. Control can be achieved by knocking the wee buggers off with a sharp stream of water, a power hose is not necessary and may cause more damage then the Aphids. I use a 1 quart spray bottle similar to one that window washing sprays come in.
The link below provides a great deal of very good information about Aphids.
Here is a link that might be useful: About Aphids
Okay, so call it a harsh water spray.