From an engineering standpoint any chemical reaction that changes the properties of a metal in an unwanted way will be called corrosion. It is something to prevent and/or monitor that will inevitably shorten the lifespan of the metal part. (Non-metals like fiberglass can also "corrode".)
Fluoride (with a 'd') is not fluorine (with a 'n'). The difference is as big as that between sodium and sodium chloride. You can eat one, but not the other.
The most powerful oxidizer in MSR fuel salt is the U(IV) ion.