What Are These White Flakes On My Plants

Never had these before. I have put my bedding plants into an plastic greenhouse and a few of them have developed dry white flakes on them. I have had all sorts of mildews and what have you in the past, and am well up on pests and diseases but never had this. A quick google says they could be the discarded skins of aphids which they shed as part of their cycle. 

Any other ideas?