The dark line at the right shows the edge of the lintel. This area would have been filled with mortar. Moisture has penetrated the mortar (and probably the brick ) resulting in the steel lintel rusting.
What happens when steel rusts
When steel rusts, it expands in volume pushing the mortar outward and pushing the bricks above and below further apart allowing even more moisture to attack the lintel
A horizontal straight-line crack will normally develop at each end no later than a few years after construction.
Below is a video that shows how to remove and replace a steel lintel. This normally does not need to be done until the house is 50 years old or older.
The most important idea to take away from this video is this: the horizontal cracks at the end of window and door lintels are due to moisture, not foundation distortion.
Now that you what to look for, watch the 4-minute video: