Java moss can be a slow grower sometimes and it seems to attach to wood easier than other things. In my 20g shrimp tank it has attached to some of the gravel I have in it to help keep other plants anchored but it was tied to wood first and then spread over the bottom of the tank and then up the back wall.
In the 55g pleco tank I have a big piece of drift wood that the pleco has hollowed out for his cave and the java moss has crept across the tank and moving up the side of it...looks pretty neat, the shrimp love to pick through it and keep it clean.
I use cotton thread to tie the moss to wood and within a month or so the thread has dissolved and the moss has attached to the wood and starts to spread.