Vanish is good for most things, but since it has dried it'll be more difficult. Spray it on, leave for at least quarter of an hour, then pad from outside in with a damp cloth or sponge. Repeat till it's faded enough to be acceptable. sorry can't be of more help.
White vinegar in water (a teaspoon to a pint) or dry baking soda on a damp cloth/sponge might also work.
Good luck
And if it's still quite unsightly turn the mattress over
or phone your insurance company!