
April 1936. View from living quarters at 730 West Winnebago Street, looking back down the alley. Milwaukee, Wisconsin. View full size. Photo by Carl Mydans.
Notice how they put the gutters on the roof instead of under the roof edge. I am surprised that it withstood the weight of ice/snow coming down the roof.
These homes were probably heated with coal, if you look at the basement window on the side of the street where the car is you can see what the coal dust does where the coal chute goes in. I think the "ghost" on the other side of the street is coal also, but how it got like that is anyone's guess.
What they meant was ghostly-looking blur that's a person moving through a time exposure. Like these ghostly guys.

In this case though I think it's just a stain on the wall.

This is exactly the sort of thing that people who frequent websites about the paranormal love. When you blow the picture up to its full size, you can see that it's clearly just a dirty blotch on the wall.
[I don't think that's quite what they meant. See above. - Dave]
Here is that same address today.
coming down the outside stairs?