Answer: This may be normal for this kind of moss. Moss is very shallow-rooted, and if it's growing in soil that is very shallow (covering a rock or a gravelly spot) the extra drainage allows the roots to dry out more quickly than mosses that grow on deeper, moist soil. I have seen moss that in the spring is green and soft, growing along the edge of rocks where they come to the surface, and in the late summer it's almost white and crunchy, it's so dry. The following spring, the moss is lush and green again.
If you suspect that something other than moisture cycles are at play, maybe a local biology or botany teacher/class can help you explore this further -- I'm afraid we don't have any in-depth resources on moss cultivation. Good luck!
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