It isn’t fog, though it looks so from far enough away. Only once you get close enough can you see it’s really insects, swarming by the billions across the hillsides. They aren’t a danger, and the colonists have long since learned to simply ignore them. They don’t bite, or sting, or devour things—- at least no things that matter.
Or, in another way, they only devour the things that do matter.
Because they aren’t gnats or mites, they’re tiny remnants of the terraforming started centuries ago. Soon enough they’ll die off, when the atmosphere has finally hit ‘Earth normal’ and the toxins they survive upon are gone.
There will always bee a few swarms, hovering around volcanoes above and below the waters, but by the fifth generation of colonists the free roaming swarms will be nothing but memories.
At least that was the plan…