Keep your moss art out of direct sun. The sun will cause fading the bright colors. Keeping it indoors an out of the elements will insure the art to last a long time.
Your moss art does not need any water. The moss is all dried and preserved and watering will not do any good to the arrangement. That being said, sometimes if it's very dry out, the moss may become hard and crunchy. If this is the case then you can lightly mist the piece.