It's difficult to tell, due to the quality of the image, there are several predators that keep aphids in check, the most common are ladybirds and their larvae, lacewing larvae, hoverfly larvae, spiderlings even. Adult spiders will also eat them, and several parasitic wasps, even earwigs. The position of what looks like legs are throwing me off IDing it. Although saying that, I'm going to say that it's quite possibly a Ladybird larvae or pupa, due to it's markings on its abdomen. More likely a pupa, as you said it hasn't moved. If I were you, I'd be monitoring it, over the next day or so, to watch it go through it's phases. On the other hand I may be completely wrong.