Near Future Sci-fi with Feudal aliens deep in Europa, and a sonar proof wetsuit.
I remember in the early chapters of this books, humanity has encountered another advanced alien species and is diplomatically engaging with them.
It is then discovered that a sophisticated but still developing underwater alien species is deep on Europa. Humans and the advanced aliens are discussing how to engage these new Europan aliens, when a human journalist decides to get a human sonar invisible wetsuit, designed for sneaking up to human submarines, and goes to Europa to investigate and take photos of the Europan aliens.
However, while the Europan aliens can't perceive the journalist, and he goes around taking photos of them and their houses, they can perceive the lack of anything there. Like a visual black hole. So they end up cornering this 'ghost' in the corner of one of the houses, and go to investigate it with their claws. Except their claws are super strong and inadvertently cleave straight through the journalist, which raises a huge amount of questions for the previously uncontacted Europans, and throws the human/advanced alien relations into turmoil.
I think the Europan aliens are also Crustacean-esque in design?
There's also a later scene where there's a group investigating a small human underwater habitation building, and they notices furry droplets on the walls, which turn out to be microbes feeding on the old bloodstains or something.
I can't remember anything else about the book, or the ending, just this one really specific chapter that I found absolutely fascinating when I read it.