Not sure what moss you have, but assume it is Java Moss. It is very slow growing (a good thing) and should be attached to driftwood and not to thickly giving it good water circulation and time to spread. Once it is established a slight trimming usually allows it to fill in nicely and although it is low light a medium light will usually grow it faster. I find that mine seems to grow best attached to rocks or driftwood. Planting it in substrat it tends to get a lot of debris in it and looks unsightly. Trimming it keeps it low and the new growth looks good. I also found my java ferns grow faster. Just be patient with it.