I am trying to remember a novel I read in the 1990s, although the book was probably much older. I don't really recall the main plot, but there was one incident that really stuck in my mind.
The episode I remember had a human spacecraft contacting aliens on a planet they passed near. The buildings of the aliens' city had wide flat roofs cris-crossed by canals. The aliens were moving in the canals, but the humans cannot see them. The aliens allow the humans to see their city, but they block the ship's sensors from seeing the aliens' actual bodies. The humans see only black spots where the aliens are.