The carboys always get a soak with homemade PBW mix of 70/30 oxiclean free and TSP/90 for 24 hours, then rinsed well. My guess is it's some kind of deposit building up maybe? But soaking it in a mild acid solution isn't getting rid of it. I mean, I don't taste anything really off in my other beers, but whose to say it's not something subtle and the dark beers roast character is more affected by it? I don't know, I'd doubt the haze is causing any issues, just curious.