Have you noticed that it does well in full sun?
I am trying to find a good short compact moss that does well in sun and heat
I am a moss skeptic...BUT the best places to look for "silver" moss (Bryum Argenteum) is in spots that are well drained, but retain moisture and that get large doses of sunlight, like between sidewalk pavers, the margins of parking lots, etc. Those locations tend to be extremely sunny, but they also tend to remain a bit moist-particularly after rains. Moss has no roots. It relies on moist, but well-drained soil.
You grow the particular species of moss replicating the environments you collect them from. You can grow bryum species by placing a brick with some finer topsoil (spread like peanut butter on bread) on a brick. Place the brick (s) in a shallow pan of water, water must come only halfway up the side of the brick. Place the container in a spot that gets morning sun for four or five hours. make sure the water doesn't dry up completely. Wait....and wait...and wait. The moss will probably begin to cover the brick, as the moisture wicks up the brick into the topsoil...
FWIW, like many moss species, the silver moss is mostly unstable on bonsai soil. Without a top soil dressing, bonsai soil tends to drain far too quickly and dry out to quickly to sustain it for any extended time. Trying to maintain the moss can come at the expense of the tree it's growing on.