How to get a stain off a mattress

[saltire]

DS2 wet the bed on Sunday night. He never told us, and I didn't realise until yesterday afternoon. Anyway, I washed the mattress, and now the washed area has dried, but its left a watermark - any way of getting rid of it?

Vanish carpet/upholstery mousse might help, but tbh if it's dry and doesn't smell I'd live with the mark.
Or buy a cheap mattress cover to hide it