I have seen some crescent scars on plums, and I have seen a lot of those scars on apple trees right next to the plums. With my reading glasses, I have seen the actual insects – they have camo to look like a spec of leaf but closer examination reveals their pronged snout.
Later in the season, I see white crystals on the plums – juice is oozing out of the ripening plum and crystalizing? I have attributed these to PC strikes, but could they be something else?