How shady is it under the acer canopy?
Bergenia, Liriope do well. Hellebores do well as well, liking the shade in the summer and flowering in late winter before the acer leaves emerge.
You might also consider Gautheria procumbens aka wintergreen or checkerberry.
It likes similar acidic conditions as your acer and should compliment it well.
It's just coming into flower right now, which will be followed by red berries.
Approx height 6".