That is what we call Irish Moss or Scotts Moss - Sagina subulata or related species - as said not a true moss, a very invasive weed. So invasive, it is sold for use as a lawn substitute where there is foot traffic, you can not "stamp it out". Don't let it near your bonsai pots, you won't be able to get rid of it.
My favorite true moss, is "crack in the sidewalk moss". I have no clue the species, but this is the one you should look for. Common growing in the cracks of concrete in urban and suburban areas. Might also be found in cemeteries, in rural areas without sidewalks. Also moss can be found just about anywhere moist. Pretty much any true moss can be used for bonsai. Collect a few different species the mix of textures will look nice.