'Dandruff' like dust on moss and medium

[YetAnotherOrchidNut]

If its aphids as shadeflower suggests (which was my thought too) check new growth especially closely. A lof of orchids are fairly robust to aphids with thick leafs and stuff like that so they home in on the sensitive new growth. The particularly like Dendrobium or Oncidum new pseudobulbs in my experience. No idea about Zygopetalum, i have no experience there.