“David, what does burning smell like?” I mean, what does rot look like?

Do you think this looks like rot? The leaves feel strong/tough but there is a bit of give when I squeeze them. Is that just because they have hollow pockets? Photo 4 and on is another one that I’m less worried about given the colour, though I give them the same care (about an hour soak roughly every 10 days or so, then they live upside down in a southern window in a jellyfish holder shown in the last photo, but they get more light than the photo shows)

Any help would be appreciated, thank you!