you can get it to crawl on the substrate, but any disturbance will peel it off with the first layer of substrate. i would get some flat thin rocks (slate works great) and attach it to that and lay them ontop of the area you want carpeted, or use stainless steel mesh squares. both are popular and both worked fine for me. however, the rock tieing seems to make a carpet faster (as the moss doesnt need to grow out through the mesh, just grow a touch to cover the string).
Eheim Pimp #296
Canon Pimp #67