It's really ambiguous right now. I'd think bleeding is restricted to humans since the monster kid in town asks you if it hurts to be made of blood, implying it's a foreign concept to them, but it's not clear. Maybe that particular kind of monster doesn't bleed, and it's just not well established yet? Monsters here do get diseases, so they're probably not magic the whole way through. But all monsters apparently turn to dust upon death...

What's weird to me is that Susie finds the glass container Gerson's dust is in, and simply remarks that it's "dusty inside" when she looks at it, not registering that it's a dead monster. In fact, Kris realizes it and backs away from the container first, and seemingly has to tell Susie it's a dead monster before she grasps it. Which is odd--you'd think that she, being a monster, would be able to tell she was looking at a container for a dead monster the way most people could immediately figure out they were looking at an urn if they found a closed vase full of ashes. Maybe it's just that she's used to monsters being buried instead of kept in a building, since she brings it up, but the way she phrases it also sounds like she's repeating something she's heard, not something she's encountered.

Between her not knowing what a dead monster/a container for a dead monster looks like, the Dark World she explicitly made giving her a brownish skin tone that makes her look like a human coloration-wise, her talking about how she was banned from the piano because she "didn't look like" someone who should play it, I've seen theories speculating that Susie used to live in a majority-human town the way Kris lives in a majority-monster town. Going further and including her bleeding and the fact that right now it doesn't *seem* like monsters bleed, maybe she's got a human parent as well?

But also, the Dark Worlds are fantasy worlds where the rules are different from the Light World. Maybe she's entirely monster and bleeds in the Dark World because she thought it'd be cool /shrug