They look like spinach leaf miner eggs to me. These are laid by the adult fly - once these hatch into larvae, they will burrow into the leaf tissue. They have 3 or 4 generations a year, unfortunately. You can rub them off, but if you have a heavy infestation, spray with neem. Rotate your leafy crops year on year; putting a net or cage for protection from the adult to stop the laying of eggs is a wise precaution before the crop gets going properly next time you plant.
Note there are white aphids - whitefly, but they, as the name suggests, fly.