Perhaps a nymph stage of certain insects, the white flecks being skin moults from a smaller nymph stage. Flies do not have these stages.

But after all winter, there should be the adult insects developed--unless the greenhouse is quite cool.

Looks like the exoskeletons of your aphids. Aphids, of course, are undesirable.

Yes, the pale objects could be the final moult exoskeletons of the aphids, they are about the same size as the adult aphids in the image. Test to see if the pale ones have any movement.