Sorry, I don't think you can, I've tried lots of stain removers on mine, and also a stain devils one that should remove mould spots, but to no avial, the marks have faded a little but they're still there, if you find a remedy I'd be pleased to hear about it!
Regards M
Why don't you just recover them? then you can take the covers off & wash them thus avoiding the dreaded mould.
Which does tend to come back unless you store them in a dry place.