Last season's mulch has a whitish...

Most mulches are mixtures of shredded wood and bark residues, and they decompose over time. Fungi and bacteria always accompany the decomposing process. Sometimes the fungi become visible. You may be dealing with a “slime mold,” which is usually bright yellow or orange and can grow from several inches to more than a foot in diameter. This type of mold will dry out and turn brown, leaving a white powder behind. If you want to get rid of the slime, you can dump it in your compost pile or household garbage.