In DragonVale for the iPhone and iPad, the Moon Dragon is a rare dragon that can be hard to breed.
To get a Moon Dragon, you breed a cold type dragon with a lightning type dragon, however your chances seem to be better when using hybrid dragons. For example, the most popular combination to get a Moon Dragon is by breeding a Crystal Dragon with a Blue Fire Dragon.
Some other possible combinations are Fire + Storm, Ice + Scorch, Scorch + Storm, Mud + Storm, Sonic + Blue Fire, etc.
Note that any of these combinations do not guarantee results, and the different combinations have different percentages for success. Keep trying though and you’ll eventually get your Moon Dragon.