I generally check all my trees that have not been repotted at this time of the year to make sure that any weeds that may have established themselves in the soil are removed. For the most part, due to the inorganic soil mix I use. this is quite simple and the roots come out easily.
At first, this weed looks quite nice, forming a lovely green undergrowyh that is in perfect scale for most trees, resembling patches of grass, but it soon becomes invasive, covering every bit of soil.
I can only recommend that you keep a close eye out for this weed, as it is nearly impossible to eradicate once it gets a foothold in your bonsai.
This plant forms a dense matt of roots in the pot, destroying any drainage and robbing the Tree of nutrients.
The only sure way of removal is to bare root your tree and wash every bit of moss root out of the trees rootball.
With pines, where barerooting is not a good idea, that means thorough checks every week to remove any new growth, which will eventually get rid of the weed.