most mosses aren't suited for foreground plants, at least what i've seen with them and experiments. this mainly has to do with how thay grow (i.e. thay just exapand when thay grow). usually you want to use something like dwarf sag in low light or HC and UG in high light.
here are two good grass like plants Echinodorus tenellus (pygmy chain sword) and Sagittaria subulata (dwarf sag). the sites have there requirements and how big thay get.