I think it may be harmful, as in cyanobacteria, even if it isnít I would scrape it up, dab up the moisture with a paper towel or tissue paper and sprinkle a little KH₂PO₄ and cut the humidity a bit.
If your Guru tells you otherwise and you are true believer that is okay I will not argue the point.
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I pretty much limit what I say to things that anyone can easily verify from recognized references or by direct observation. If it is my opinion I say so.