The main thing is that the leaves are still s good healthy green. There are creature out there nibbling on everything at the moment - it's what they do. I expect it's a small wasp larva or similar, and won't harm the tree's overall health (unlike the potential sickness aphids can spread). Keep the tree well watered and fed and it will ride out the storm.

Looks like a plum leaf spot, would certainly give it a spray with something like Roseclear. Although it is fungal disease, Roseclear has a systemic insecticide too, which will deal with any lingering pest that might be present.