That sounds like the hooch that develops when it's hungry, Rebecca. You can either stir it in and feed it or pour it off and feed it. If you keep it in, the flavor will be sourer. If you're curious, we have a blog article on sourdough troubleshooting that shows a photo of just such dark liquid on top of a hungry starter. Annabelle@KAF
August 29, 2018 at 2:42pm
In reply to Should I be concerned about refrigerated starter having a very … by Rebecca (not verified)