Generally speaking, poop isn't pleasant, but it also isn't exactly lethal. But every year jellyfish unleash bowel movements so deadly that they can destroy entire marine ecosystems. A little odor doesn't seem so bad all of a sudden, does it?
Jellyfish are expanding into lots of new waters due to a combination of global warming and overfishing, which together has greatly reduced what few natural predators that these creatures have. With pretty much nothing to stand in their way, these jellyfish undergo massive birthing periods in the late springtime. These are known as blooms, and it means thousands of new jellyfish appear in areas like the Chesapeake Bay every summer.
Here's why that's a problem. The jellyfish eat organisms like zooplankton, which among other things help convert carbon into solid material, which is known as carbon-fixing. The jellyfish digest these plankton and then poop them out as one big gelatinous, carbon-heavy mass. This in turn creates massive amounts of food for certain bacteria that would otherwise be pretty much nonexistent. These bacteria go through a feeding frenzy, converting these carbon blobs into carbon dioxide.
This has now removed huge amounts of carbon from the water, which all the other marine organisms need as a food source. The runaway jellyfish bloom has cornered the resource, and the most common result is fish starvation. There's no obvious solution to this. Perhaps some enterprising soul can write Everyone Poops for young jellyfish...only, you know, try to make the exact opposite point.