These do look like white flies to me or something similar. There are very effective organic insecticidal soaps that I think I would try first thing. Are these in your greenhouse? I would look and keep watch of all other plants in there. Even if they show no signs now of infestation, the problem may go looking for new homes if you make this one inhospitable.
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~Tina


Drama Free Zone.
What every gardener loves the most, Begins and ends in rich compost. (Tina)