A photo would be helpful Annette

This is possibly capsid bugs, it's the excreta from them. Try washing the leaves off with soapy water in the evening before it spreads to the whole tree.

It may be aphids secreting their waste, honeydew. Taste it, probably sweet. Just wash it off and look at the newest leaves for aphids. On my acer palmatum 'Bloodgood' the aphids go pink, because the acer's red.