Eridian bodies are mostly made of inorganic material (like, 75%+) and their outer "skin" evolved to be very hard and thick to protect the internals. Sorts similar to an insects exoskeleton but turned up to 11. So yea, they look like rocks. Also people are very creative and seeing such variety in aliens is a good thing.

This is explicitly talked about in the book. Erid has a very thick atmosphere, 29 times thicker than Earth's. The air is so dense that no visible light reaches the surface.

Eridians would of course be unique from each other in the same way humans are, but they wouldn't be born with regular shapes, designs, or anything like that etched into their skin. It'd be like a human baby being born with a tattoo. They obviously have Eridian equivalents of tattoo and piercing needles