Early in the book I'm writing I introduce my heroine's ability to turn into a ghost basically at will. She can't bring anything with her in this form, so anything she's carrying or wearing falls to the floor. But this also applies to things like dirt that's stuck to her body. I want to demonstrate this by having the marker ink she accidentally got on her hand earlier in the day fall off of her after she ghosts. But what would that look like? Would it be liquid-y? More solid? Would it still be in a line when it falls off or quickly fall apart? I honestly have no idea.
I think I'd just have it vanish. Unless the entire back of her hand (or palm or whatever) is solidly scribbled with it, and probably even then, it's such a small amount I don't think you'd see anything fall away. probably the equivalant of a very small drop of ink, but broken up. You'd almost need a microscope to see something like that. Though you might have her wonder in fascination what it looks like. Or actually, the ink might end up very faintly smeared onto her clothing that fell on the floor. But it would be such a tiny amount, and it would be dry by that point, so would act more like say a charcoal smear rather than a liquid ink stain. But it would show up on white or very light clothing if it happens to get on that.
How about the colonies of bacteria and other micro-organisms that live on the skin, especially in the dead skin layer? Does that all fall away, too? Or go with her, as part of the ecosystem that is her body. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535073/#:~:text=Microorganisms%20(viruses%2C%20bacteria%20and%20fungi,in%20the%20hair%20and%20glands.
The metahumans of my story don't quite have the same relationships with micro-organisms that humans do, so that's not an issue. Food *will* ghost out of her stomach if she hasn't taken about a half hour to digest it enough though.
Xoic makes a good point about the marker. I'd suggest going less subtle. You mentioned dirt, why not that? She could have gotten exceptionally dirty at some point and the dust sort of puffs away from her when she ghosts. You could go full Carrie and soak her in blood. Or maybe something less extreme like paint? If you're gonna' show it, make it a show.
