Acer leaf spot
Acers especially the varieties with feathery, lacy foliage always benefit from a sheltered spot in the garden. But sheltered spots with poor air flow can also be wonderfully kind to mould and fungal infections. If you start to see your Acer leaves with similar discolouration to that shown in the image you are looking at downy mildew. It’s nothing too serious but it should be treated with a broad spectrum fungicide which will be available from any garden centre.