Hi Jane and welcome to GoY. Could you be a little more specific or add a photo to your question there are lots of 'white substances'.

Do you mean a white substance on bark chippings?

The community vegetable garden had white all over grass clippings used as mulch.

I wonder of she means lichen. is it like a crust growth on the bark? if it is then it will be lichen. it means your air quality is good. it wont harm the plant at all.

Could be a fungus helping to break down dead matter or one you don't want if its on a living tree.