Scale insects, so called because they look like scales on the plant. How bad is the infestation? Remove and destroy any really heavily infested leaves and apply an insecticidal soap or oil containing neem. Be careful with oils and soaps in the glasshouse as the heat and light combined with the chemical can damage your plants so don't overdo it. Be persistent in removing them. I had them on an ornamental palm and they are quite difficult to eliminate completely. Once you have gotten the infestation under control with a spray, scout the plants regularly by inspecting both top and bottom surfaces of the leaves. If you spot the odd one or two you can scrape them off with your fingernail (or other implement) and squish them or put them in a bucket of soapy water to kill.