Many tree leaves can get various molds and mildews and most cause few problems for the trees. If the leaves become covered over more than 50 percent of the leaf surface that could inhibit photosynthesis enough that the tree will drop the leaves.
Whether what you have is something of concern probably is best addresses at your local office of your state universities Cooperative Extension Service.

If the tree is the kind that loses its leaves for the winter (deciduous), realize that it's pointless to treat leaves which are due to be shed in short order. Here, the preferred management is wait until the leaves drop, then rake & discard.