I bet that the slide is not "slightly faded." Rather, it's coated with a very thin wash of diluted white nail enamel.
It's worth trying to use some straight acetone, or some more nail-polish remover, and a soft cotton cloth, to scrub the rest of the enamel off.
If the metal was originally blued, this will not remove the blue color. Rather, it should restore it.
But if the slide was originally painted, or otherwise coated, you may do some slight damage.
However, that damage would be merely cosmetic, and not functional.
Next time, use either an ordinary white crayon, or buy the correct material (I believe that it's a type of shellac stick) from Brownells.