Buy a box of white fondant. I say white because if you don't really want all white ghosts you can add coloring to have colorful ghosts, or even tie-dyed ghosts.
Follow the directions on the package. Knead the fondant until it becomes soft and then roll it out to 1/8" thickness. I sprinkled a little corn starch on my marble slab to keep it from sticking. (I also managed to sprinkle it across my face. I've been told I'm a messy cook.)
Then, simply drape the fondant over the marshmallow to form the ghost. For the 2-stack of marshmallow, use a toothpick to hold them together. They begin to harden in a short time.
For the eyes and mouth, use a toothpick to place dots of black frosting where the faces should be. Allow them to dry.
I'll admit that I did not eat one, as I am not that fond of fondant (although I ate plenty of marshmallows!). I suspect they are probably better as a decoration than a snack, but I'd bet a kid wouldn't mind one way or the other.