Remove from foam ,soak in mild soap solution.Rinse throughly .Air dry and re- proof with Fabsil gold .
If cushion is not removable ,You could try using a damp sponge,soaked in fresh water .
.Damp stains usually disappear on their own when dry .If stain remains its usually not just water that has caused the stain .Try sponging with diluted wash up liquid from outside of stain towards centre .Allow to dry .