Is this PM?

When steam is produced by a humidifier, as with all water steam, it is devoid of any mineral salts and this is how they make distilled water, which is regarded as "pure" due to nothing in it but H2O, so for any substance to collect on your leaves due to using tap water in your humidifier is physically impossible. So it is more than likely a mildew or mould of some sort, no doubt growing on the leaf due to excess humidity within your grow space. Cannabis does not really need or like a high humidity environment as it is not a rain forest plant, and this fascination with humidifiers, for better or worse, seems to be a quirk related to American cultivators who seem to believe in it like it is the gospel truth, without ever asking if it is really necessary. In Europe or at any commercial seed producing facility, you will never find a humidifier, lest it cause a massive mould/fungal problem and destroys a whole crop of seeded plants. Trichome production actually is better in an arid or very low humidity environment and the only time cannabis needs boosted humidity is for the benefit of cuttings while they are waiting for root formation. So, save yourself some money and ditch the humidifier, your plants don't really need it and the risk of inducing mould in your buds by having too high humidity is too great. Hope this helps,............ Organoman.