The die-back world in Trojan Reach, or the water world in the Solomani Rim? Apparently there are two Noricums - seems unlikely, but maybe it's a common word in the far future.
The water world is easier for me. Low grav, thick atmosphere and abundant aerial lifeforms reminds me of something from the Vorkosigan universe. A native gas-bag floater "jellyfish" (not really, but close enough as a description) with dangling stinging tentacles, almost invisible in poor lighting, and with such a different biology that it tends to explode and rain stinging tentacle pieces when hit with stunner fire.
Trojan Reach is probably more likely, but I don't actually know that one outside of Travellermap's wiki stub, so no ideas yet.