Hard to tell from the photo but can you scrape them off or are they, like, attached? I had something very similar on my habanero seedlings and whatever it was was a part of the leaf, like they had grown with it. I swear it was some physiological reaction, and when I planted the plants out the new leaves didn't have it.
But anyway, if you are able to remove them you should with a strong jet of water or even your fingers. If you feel you must Neem/soap treat, be very careful not to do so in the full sun, especially in the greenhouse, as the leaves can get badly damaged when the sun hits the residue on the surface of the leaves. Also you should keep a close eye on the plants when in the greenhouse as even a mild temperature outside can quickly build up heat inside and cook the plants.
Good luck with the aphids :)

It looks like oedema - caused by over-watering/high humidity. Cut back on the watering and maybe put a fan on the plants to lower the humidity.