Or it could be something picked up along the way that was a little oily, maybe something from your brew pot or maybe one of your additives. My stout looks like that as well. I wouldn't be worried if you can't taste any off flavours. But yeah maybe try rinsing for a little longer next time. Does the beer have good head?
But I am fairly new as well, best to wait for someone else to chime in.
I know nothing!