How come when i see fluff and i think it's a spider, i'm scared, but when i see it's not, i'm not scared even though it's like a spider?

Because you're scared of spiders. When you glance at something and you can't see it clearly right away your brain fills in the empty spaces to something that's familiar to you. In this case, a spider.

Likely the thought of a real spider near you frightens you. not uncommon at all. When I see dust bunnies, I can readily tell the difference and if I find a spider near me or even crawling on me, I am relieved that it is just a harmless spider. They are not the bad guys that go around biting people,and worth having them around.
Not knowing or not being familiar with spiders can incite fear because of all the horror stories around.
People seem very afraid of bite, but as a rule, spiders just don't bite unless trapped or smacked against the skin. When they do bite, they are usually insignificant nuisance bug bites.
I don't know why arachnophobia is such an epidemic these days, People seem too preoccupied to learn a little about our own natural environment and these fascinating critters.
Maybe get in the habit of when you see a spider indoors, just say hi to it as it goes by, it certainly has no intention of bothering or biting you.

idk that makes no sense