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Did you spray something on the leads with some old fertilizer in it? Happened to me once > sprayed water that had some rooting hormone in it by mistake and my leafs look like that till I sprayed a few times... Could also be Powdery mildew

Yes as others have said - it’s from some water or solution being on the leaves and drying and leaving reside

Residue, as in stuff in water or the remnants of a foliar spray or pest prevention you may have used. That's what it looks like to me, as well.

Possibly residue from minerals in the water.

Μου έκανε εντύπωση.Δε νομίζω να είναι κάποια έλλειψη.Οταν λέτε υπολείμματα τι εννοείτε? Τι μπορεί να έχει μείνει πάνω στα φύλλα?

Not the best picture and without a diary we don't really know what you got going on. It looks like possibly residue from a humidifier or residue from getting the leaves wet and it dried.