Answer: Are you sure this is a fungus? It sounds to me like it may be leftover fertilizer pellets. Many greenhouses and nurseries use a pelleted, slow-release fertilizer in their potting mixes that fits your description exactly! (Other people have wondered what these pellets were.) If you've used nursery-grown plants I'd bet that's what they are. Just leave them in place to do their thing--they'll break down slowly and release fertilizer.
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