I get the same thing under my wedding band (14K gold) and my watch (stainless steel back and band). It also happens to my entire hand whenever I wear rubber gloves, which is pretty much every day I work.
It’s sweat. More accurately, it’s sweat that gets held against your skin for a long time with no air circulation to evaporate it away. Like a stagnant swamp, all sorts of funky, smelly stuff grows in that stuff.
Why do you think I wash my hand regularly? I also move my wedding band around a little so that it isn’t in exactly the same place all the time and take off my watch sometimes. That allows a little drying, a litle airing, and makes it easier to rinse the funk off in a sink.