Does this foundation wall crack justify immediate corrective action?
From the looks of it, you live in an old house.  From the pictures and your description,  you have a stone foundation with brick walls.  Where the wall narrows is where the brick sits on the stone.  The walls in the basement(stone and brick) were parged.   It looks like you have a superficial crack in the parge coat.  I would scrap all the loose material off, and repaint with a thick paint intended for basement walls.