My experience with Java moss is that it is extremely invasive on anything it can cling to. Don't take this wrong, I like the stuff, but I find it to be a real PITA to control. if you spread it out on/in your substrate, it will grow in every direction including up,you can trim it pull it, cut it, smash it etc. and in a few days there will be more growing somewhere nearby. it would eventually fill in all the blanks, but you will need to keep it pulled off of the plants you want to grow above it, and you will need to keep it trimmed short to prevent it smothering out thoses same plants. Don't let me scare you the stuff is easy to remove from most surfaces, but it really does grow everywhere it lands. And as said it is a decent mechanical filter, but once it starts growing, your last concern will be whether or not you uproot it. I try really hard to uproot mine every week when I vaccum, even when you think you've sucked it all out a few days later there it is growing again.
Dave