Skim the dirty stuff off, let the head reform, skim a little fresh yeast and propogate it on in a pint starter.
Shake regularly, let it ferment and sediment out, pour off the spent starter medium and add a little preboiled cooled water. Skake it up to suspend it and store in sterile containers. (You can buy Sterilin screw top sample vials from the chemist for 30p ea which are ideal).
They will keep for 6 months or more and will require making into a 1L starter before use (bigger for larger batches or higher gravity beers).
(btw you can still harvest from the slurry if you prefer)
