These are called whiteflies (entomologists are notoriously skilled at naming their insects), the most common insect pest in the greenhouse. There are a number of different kinds - greenhouse whitefly, silverleaf whitefly, sweetpotato whitefly, banded wing whitefly, etc. - but the bottom line is that tomato plants would be better off without any of them. Whiteflies suck the sap from plant leaves and, in the process, can spread some very serious viral diseases that infect tomatoes, peppers, and other plants.