Oil and water don't mix. If your container wasn't tightly closed you might have got some condensation in it. But it would just sit on top rather than producing a lighter colored layer. I have an old Corvette that gets briefly driven every ten years. In 2008 the oil was changed prior to it being driven. In 2018 that oil, with maybe 5 miles on it, was drained and replaced. I didn't notice any change in color due to settling as it drained.
I wonder if your opened container got some used oil or even brake fluid in it without being noticed or remembered. As stated, for peace of mind change the oil. On close examination, my Corvette's oil is only slightly black, not nearly as dirty as used oil gets. So I use it in lawnmowers, etc.