is it powdery -- could it be mildew? or soft and fluffy such as mealybug? ones a fungus and one a bug so will need different treatment

We have this too, and if it isn't fungal, I would say it is caused by scorch from the sun where the leaves are wet (possibly from drips from the greenhouse roof) and then the sun shines on them.
If that's the case, they will be fine once the weather warms up and the greenhouse is dryer or when you can stand your trees outside.