i dont know this species off hand, i may have a seedling but I am no expert.
BUT, i would switch that moss to OVER the roots and put the roots on the slab...then they are kept moist and they can attach, here you are letting the moisture leave and encouraging the roots to attach to the moss
i'll see if i have one and let you know but if i do it is mounted upsidedown with a little sphag over the roots or completely bare