Mixology - a place to share and discuss cocktail recipes

When pouring a drop or two always goes over the edge onto the threads of the opening. I stored the bottle in the pantry for a while then later decided to start keeping it in the fridge (because I assume a chilled Baileys will be less dominant in a baby guiness so the coffee taste prevails more).

I noticed a nasty charcoal colored slime building up around the neck/mouth of the bottle. It’s a black bottle which hides that, but wiped it off with a napkin and saw the black color. Of course that also means that whatever that black stuff is, it mixes with whatever I am pouring unless I wipe it off every time before pouring.

WTF is it? Ingredients of standard Bailey’s are:

Irish whiskey, cream, sugar, and a blend of cocoa and vanilla flavors.

So I wonder if that is not spoilage, perhaps it’s just cocoa and vanilla additives. Or perhaps the aluminum cap reacting. I’ve noticed that dishwasher-unsafe aluminum turns black in the dishwasher. So maybe it’s the cap reacting with something in the Baileys?