Answer: You're dealing with a standard wood rot (Basidiomycetes). The best way to stop the growth is to expose the mulch to fresh air, which kills the wood rot fungus. You can rake the mulch to move it around and help dry it out. Doing so will expose the underlayer to fresh air and sunshine. If you're stockpiling the mulch material you can spread it out and then restack it. Try to keep to it turned and moved to prevent the spores from developing and spreading to the rest of the mulch.
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