Covering Exposed ICF and sill plate?
You could install a trim board, but that doesn't adequately address water drainage. You'll end up with water behind it, rotting out your sill. 

I would do your stucco/parge coat, then install color-coordinating Z-flashing over the sill plates. The flashing should be inserted behind the siding channel (and any housewrap or other flashing), so you may need a custom bend to accommodate your dimensions. It should lap onto your stucco a reasonable distance (say an inch). Ideally it will terminate with a hem for stiffness and a finished appearance.