I noticed that the cement covering over the foundation on the West side of the house was coming off. We had this problem once before. The covering is called "parge" and is basically just a layer of masonry mortar to cover the raw cement foundation.
So I called Dean's Plastering and Stucco Repair (which I found listed on NextDoor), and asked if this is something they can fix. They came out a couple days later to fix it.
First thing they did was remove the part of the old parge that had separated.
That showed the extent and cause of the problem.
The cause of the problem was rusting rebar in the cement foundation. Apparently it is too close to the edge of the foundation, and water had caused it to rust, causing it to expand. The pressure from the expansion then fractured the outer layer of the foundation and the old parge.
So they removed all the old fractured concrete and parge, and treated all the exposed rebar with an anti-rust agent. Then they applied a layer of mortar mix (cement and sand, Type S) over the top to recreate the foundation.
After letting that dry a bit, they then topped the entire area with a new coat of mortar mix to create the new parge, blending it with the pre-existing parge.
The cost came to $400.