So they must emit some sort of low frequency sound that is able to influence people's brains. It's not that the affected people can't see them, it's that their brains have been, like, pushed into not seeing them. Yuki was able to start seeing them when she actively went out of her way to avoid her "preconceived notions" while drawing. Thus, she ignores what her brain is telling her and sees exactly what actually appears in front of her. Baseball man probably went through something similar, completely emptying his mind to focus entirely on the baseball or whatever. And they can still see them without getting affected again because they haven't actually gotten rid of the brain-influence, they're just ignoring it.