Your coin is a bronze issue with a silvery appearance... this happens sometimes.
While not completely pertinent to your coin, I will quote what I said about silver on the surface of a coin --
There are other ways a coin could have a silvery surface. Corrosion may also promote a silvery surface on a coin. Lead corrodes very easily, and as it is displaced, silver can be deposited on the surface. Various methods in the cleaning process can also give a coin a silvery appearance, such as heating a coin or even washing a coin. Lead can be removed by prolonged washing, leaving more silver on the surface.
I have had a coin look more silvery after heating, perhaps something similar happened to your coin, or it is simply the result of the alloy mixture.
I will also post a coin of Constantius II
that has a very silvery appearance-- but it is a bronze coin.