I have this happen sometimes with darker clay bodies. As was said above, it's usually scumming; but it can also be dust / detritus loose in the kiln from pieces from a white body, or even kiln wash if you're unlucky.
I'd say about 80% of the time, I can fix it with some vigorous sanding in the bisque stage. Obviously take proper precautions to not breath the dust from sanding.
If this doesn't work, then your best bet is to cover it up. It can absolutely show in the final firing.