I don't know if it is still available, but many years ago I bought a product that removed mildew/mould from fabric, it had Oxalic acid in it and came in a pen form.
I think you are more likely to find it in a small shop that sells anything from soap powder to galvanised buckets, than in a supermarket.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix general clothing.