I am a succulent known for my unique, almost geometric leaf arrangement. My leaves often have a powdery coating and can change color depending on the light. I am often called a 'living fossil' due to my ancient lineage. What am I?

1. My leaves are arranged in a spiral pattern.
2. I thrive in arid environments.
3. My powdery coating helps me reflect sunlight.
4. I am slow-growing and long-lived.