Vinegar?

Actually just read a life hack article for material which suggested soaking in heavy salt walter solution, let dry slowly so the salt crystals dry and as the salt dries it is supposed to force the mold out of the material, then rewash. Have yet to try on towels, which always seem to develop problems, so unable to write on the usefulness of this solution.

Wash in a weak solution of bleach. This may be enough to remove the mildew and not bleach the cushion color too much.