Answer: The white stuff is quite natural - it's part of the decomposition process of the mulch material. The technical term is wood rot fungi (caused by Basidiomycetes). This white fungi is responsible for the eventual break down of the tissues of decaying wood. In your case the decaying wood is probably bark chips or pieces. You can get rid of it by turning the bark so what's on the bottom is moved to the top where sunlight and air will kill the fungi off. It's a natural process, however, and the mulch moved to the bottom where moisture collects and there's little air or sunshine will begin to develop the same white fungi. If you keep turning the mulch material over, you'll slow the decomposition, but eventually the fungi will overtake and break down the organic material.
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