Summary
Basically what it says on the tin, these worlds are made almost entirely out of crystal. The crystal ground tiles here will yield precious silicon when tossed into the extractor. Lunari Crystal trees also dot the landscape and can be easily identified by the semi-transparent "bubbles" on the branches. Chopping these down will drop Lunari Shards which can then be combined into the full Lunari Crystals to be used in crafting gear.
Appearance
Colors can vary pretty wildly on crystalline moons, but unless you're in a sub-biome, everything you see will be made of crystals. Even the trees and "plant" life are made of crystals. You'll also encounter ice pretty commonly as well as plenty of Organic Soup that will restore your health.
Dangers
While not recommended for a spacefaring adventurer that is just starting off, Crystalline Moons tend to be relatively safe. You will occasionally have to deal with ice storms and the "bouncy" buff from the Organic Soup might cause some problems, but otherwise the moon's denizens are the biggest threat. The moons tend to fall around an "average" threat level, making them fairly accessible without endgame gear.
Codex description
Sure, they are really cold. Sure, they aren't comfortable camping. But they are a rich source of silicon, crystals and that sort of thing. Watch out for the crystal rain, though. Ow.
Notable Features
The occasional underground deposit of Cinnabar, Prisilite and Trianglium makes visiting a crystal moon even more profitable.
Possible Weather
Crystal Storms, Severe Crystal Storms, Healing Rain, Lightning Storms, Small Meteorites.