I've used cotton thread before. Let me check something...yep, cotton thread that's been in my tank for maybe six months is still intact. I used brown and you can't see it unless you're looking for it.
The moss growing in the community tank has actually been growing really slowly. I started with about....a foot long heap about 2" thick and 4" across on top of my rocks. It had grown out to maybe 18" long, 6 or 7" across and about 5" thick. I just yanked a bunch out to stuff into my twiggy "tree" in the tank so now I've got much less of it on the rocks. However, I put some in my guppy breeding tank not long ago and that stuff's growing *much* faster. I can send some out to people now if they'd like, or if you give me a month or two I'll have bigger clumps to send.