If your sourdough is not fed for longer than about a week, a usually dark liquid can settle on the surface. This usually smells somewhat strongly of alcohol or nail polish remover. This so-called fusel layer is fortunately not a sign of spoilage, but that you should urgently feed your starter. If the yeasts in the sourdough are not fed with fresh flour for a longer period of time, they begin to starve and produce fusel carbon. You can simply pour off the greyish liquid and then feed your sourdough as usual.
