Water with distilled or RO water when the moss is dry. Cushion prefers to dry out a little before being watered. You can tell when your moss is ready to water when it is pale and dry to the touch (DO NOT SMOOSH).
When kept in containers, or terrariums moss wont dry out as quickly either due to surface area or higher humidity. You may want to put your moss pot in a cloche to keep it moist for longer especially in you plan on traveling. In higher humidity cushion moss will become long and sparce, if you do not like this appearance be sure to not keep it covered for extended periods of time.
At Moss Design Gallery we use grow lights for all of our moss products, ensure the lights are between 6,500k - 7,000k in color for 10 hours a day OR you can use a east facing window! Make sure you don't leave the grow lights on all the time, the moss needs to rest!