Poss lilly stain as I did wear to a wedding.but I think it is rust from corsage pin. It is a tiny mark but as it's white ,sticks out like a sore thumb.Going to try stain devils today,if that doesn't work will have to wear corsage or brooch...Shame .
If stain devil doesn't, work try a denture cleanser called denticlens. I have had a bottle for years and use it mainly for getting yellow marks off old linen. Just dab it on and wash as usual. Works every time