Incomplete fixing! The fog you are seeing is unfixed silver halide (i.e., the unexposed part of the emulsion that was supposed to be dissolved away during fixing/washing). Refix in fresh fixer as soon as possible and keep the negatives in the dark until you do so the unfixed portion does not print out and leave silver fog behind when you do refix.
This could also have been caused by the film not being in contact with the fixer during fixing (touching tank or itself). The fact that the image developed alright leads me to believe that you were using exhausted fix or simply did not fix long enough. However, I may be wrong here; it may simply have been misloaded on the developing reel. If that's not the case, then in the future, use fresh fix and pay attention to the capacity and shelf-life given by the manufacturer.
Best,
Doremus