Acid? On wooden shelves in shed - how to fix?

Hi, I am wondering if anyone could please help identify the white powder substance that is appearing on the shelves on my shed?

I think it may be some type of acid? It appears to be eating away at the metal of my shelves. I have previously brushed the shelves down to clear this before but it has returned, so I suspect I may need to use some sort of cleaner?

There was a leak in the roof for a few months and some tools became rusty as a result. The leak has been sorted but I wonder if it could be related to the powder substance.