One way would be to:
1) Create a green stroke/nofill Rounded Rectangle with the desired Radius of the green bend and a Stroke Weight equalling the desired width,
2) Object>Path>Offset Path by the Stroke Weight/width,
3) Change the Stroke Color of the outer Rounded Rectangle,
4) Create a path crossing both Rounded Rectangles where the paths are to end and Object>Path>Divide Objects Below and delete the unwanted parts, using the Direct Selection Tool (at least) for the straight segments from the cutting path.
Select both paths. Drag and drop them into the Brushes palette.
In the resulting dialog, select ArtBrush.
In the ArtBrush options dialog, set the direction to upward.
Draw a circle. Delete half of it.
Apply the Brush to the semicircle.