There is no "One" thing that causes moss. It is a common misconception that moss grows in acidic soil. Acidic soil COULD be the reason that your grass isn't
growing well, which would allow moss to move in. Moss is a creature of opportunity. If given a chance to grow, it will. If conditions aren't right for turfgrass to grow well, ie. shade, moisture, poor soil pH, etc., the moss will establish itself.
Pagarden, what can you tell us about the conditions where it is
growing? Is it where you'd like grass to be
growing? Is it shady? Too wet?