Couldn't it be either, depending on the circumstances. My understanding that the difference between a fog and a mist hangs on the reduction in visibility, with a common cut-off being 1km, with a fog reducing visibility to less than 1 km, although the definitions vary depending on country and situation (e.g. aviation, driving, and so on). As I've been in the Blue Mountains when the visibility was only a few metres, it would seem that this would qualify as a fog. Of course, mists occur too.