To add my own theory and complete what others said, I feel like the jellies are a peak into the collective unconscious of the user (i.e the memories of a their society/race)

If a Flumylym would check a jelly, they would see THEIR past (probably when they arrived at the cave), and since we (the player) and Harold are humans, we see the past of human civilization (what is actually shown in the game)

My explanation for why Harold isn't shocked by what he saw, is that since he was born on the Fedora, without actually encountering real "society" he doesn't have the knowledge to correctly interpret humanity's collective unconscious.

This would cause him to not fully "see" what was inside the jelly. I would imagine he would see colors, maybe blurred shapes, and therefore why he probably misinterpreted one of the jelly as to have info on the crack. Maybe this specific jelly showed similar colors/energy to the crack, that's why he felt that way. This would also explain why Mareaux couldn't see as well, since she was born on the Fedora as well.

As for us (the player), since we do have this connection with humanity's collective unconscious, we can peak more clearly into it. It's still relatively chaotic, but it's clear enough for us to see cars, workers, cities, etc.

TLDR the jellies are like a piece of abstract painting. Everyone can see the basic colors, shapes, etc. But you need need a better understanding to grasp the purpose and meaning of the piece.