mysterious powdery dust throughout greenhouse

[sevilliana]

I have noticed a very fine powdery white dust on my orchids - many different varieties. It is so fine it is almost like a mist. when I remove it with my fingers it looks and feels like face powder. It is softer/finer than that associated with mites. The only change in my greenhouse is the addition of a hydrofogger that I now run 24 hours and which is controlled by a humidistat.

Any ideas what this could be and how to get rid of it? Many thanks