What do you use in the middle of your balls? I attempted one a while back with sphagnum moss that I had left over from some carnivorous plant stuff, it kind of worked but there was a lot of debris, maybe the moss was too cheap a quality. Those look really nice though, I have a bunch of floating stems in one of my tanks and this might be my next project.
I managed to reduce the amount of debris coming off my wabi-kusa by submersing my sphagnum clumps in a bowl of water and shaking them out both before and after wrapping them with thread. It was also helpful to wrap them again once they'd been planted and sitting in water for a week or so.