i wanna type up some stuff about a personal headcannon of mine: quantonium "fever". Honestly the name is more a nod to "uranium fever" than anything, but it's more of a delirium caused by unprotected exposure to the element, that effects a minority of most species populations. Basically any species - monster, alien, human alike - can be effected by this, it's purely a random chance of genetics.
This is also partially based on Gallaxhar's "sad backstory" that gets. y'know. interrupted. There's a point where he mentions what most people refer to as like, his romantic partner? it's been like a year since i saw the movie. Giant Belthar maybe? and that he did some space traveling with her, and then they got in an argument. Honestly, this whole thing isn't a headcannon as much as me explaining Gallaxhar's motivation and making a new Thing to subject ocs to ! :]
The idea is that Raw Quantonium (such as the almost liquid-looking state, and when it was still in the asteroid, basically any "unfused" or otherwise "pure" form of quantonium) can cause hallucinations and delusions of grandeur in those who have this genetic susceptibility. The main syndromes are seeing a "Fairy", "Ghost", or other apparition that haunts you. It can look like someone you used to know, but it's usually pretty clear it's not the same person due to it's pale and translucent form. If Gallaxhar was using human terms instead of what he was used to, he probably would have described it as "a giant fairy", as it's usually something that appears at about the same-ish size as you would be, give or take about a foot range in size - not that it would have a set size. In the beginning, the "Quantonium Ghost" usually just is kinda There, unsettling you by just being there. Everyone's experiences after that point differs, but it usually boils down to the ghost pressuring you onto some "Great Quest". For Gallaxhar, it was "saving his world", then "saving his species", which lead him to him destroying his world to repopulate another and "Start anew". Once he lost access to the Quantonium, the ghost faded, but the poisoning of the mind remained, and his plans to repopulate the planet became more and more warped as time went on, with or without the Quantonium by his side.
Some other aspects are Paranoia, an inflated sense of your impact on the world around you ("every action i take has an effect on the universe even if I don't see it" positively or negatively), and violent tendencies. Most afflicted by this condition will also self isolate, though the reasons change from person to person.
I'd also like to note, in a "fused state" (such as Susan's case), the Quantonium is usually muffled enough that it won't cause Quantonium fever. Quantonium is the only "Energy Element/Compound" that's documented to have this, and even with this, it's considered one of the more tame side effects out of the main six energy elements documented so far.
