They start off formless, lost to the darkness, but when you're not looking at them, they change. They take on the appearance of the most important person in the viewer's memory, and move into line of sight. They aren't directly harmful, but are completely believable, as they are based on the memory of that person. They will attempt to lead the viewer into the fog, on a basis of lost items, or trapped friends, or any other reason.
Once in the fog, they vanish, leaving their victim stranded. They will seldom approach groups. If they are found by others, they will be seen by them as their most important person--unless they have touched the first victim, in which case they take on the appearance of their memories to all.