I would carefully go over the stain with a light solution of baby bottle steralising fluid (like Milton). Put a little on a cloth and gently rub over affected area. Make sure you wish it out thoroughly.
If it is pure linen, gentle bleaching followed by hot washes will shift it without harming the fibres - succeeded with granny's linen table cloth which had grown mould along the folds. But do it in several goes, don't expect it all to life at once, and if you can dry out in the sunshine then even better.