Freestanding wall foundation subsiding from erosion.
That wall appears to be built using concrete masonry units specifically designed for use in retaining walls. You can tell by removing one of the units and seeing if there is a slot or similar connection between the upper unit and the one below it. I can't tell for sure from the photos, but it does not look like the blocks are retaining any soil, so the wall appears to be decorative rather than actually a retaining wall.
In any case, concrete block walls do not normally require a concrete footer, you simply excavate down to mineral soil (remove topsoil, all organic soil) and begin the wall. Each block manufacturer has specific recommendations for building with their block, which you should follow. Course if you are not actually retaining soil, then it does not much matter.
As for filling in front of the wall, sand, gravel, or a mixture of the two is good, and you can put a layer of topsoil over which you can plant grass on.