That looks like powdery mildew which usually appears on plants that are stressed by drought … I see that your courgettes are in growbags … although the manufacturers recommend them for such use, I find that the amount of space and compost is inadequate for them. However we are where we are, so what I would do is ensure that they are kept well watered. Courgettes need more water than tomatoes. There are different sorts of powdery mildew and they tend to be host specific so your tomatoes are likely to be affected by the mildew on your courgettes. 😊
'The power of accurate observation .... is commonly called cynicism by those that have not got it.
George Bernard Shaw'