You've fit that wood into the rockwork pretty perfectly.
Traditional scaping would suggest that hairgrass be in the back. I think it would be pretty interesting to use Rotala macranda or Alternanthera reineckii, but the leaves might be a bit too large for it to work. Same with using Staurogyne repens in the fore or midground. Perhaps Blyxa japonica and Rotala rotunifolia or walichii would be better.
Do you know what kinds of mosses and epiphytes you're looking at?