White token in Catan cities and knights
It's a spare piece. Usually these kinds of tokens are printed on a sheet of cardboard that's big enough to fit in the game box, and if there's a bit of extra space then a few extra things might be put on the board to fill the space. Those tokens can then be used as replacements for the main tokens - so if, for example, you lost the green one, you can just state that the grey token replaces the green.