Depending on where you live and the moisture, I think that irish and scotish moss both would spread, as would native moss that you can transplant onto the rock. Many of the sedum's will also grow in the shade, but sometimes don't get the color they do in the sun. I know I have some in the shade, but it lost its red tones.

Woolly thyme

Maidenhair spleenwort (Asplenium trichomanes) will grow in a moist, well-drained shady stone wall if limestone is present, but it spreads slowly. Sedum ternatum is native to Ohio woodlands although I have never used it in a wall, but it is small enough.