“That white stuff” is formally known as albumin. While it is unattractive, it is not harmful.  It is just a protein that coagulates when it is heated. It will always make its way to the surface of the meat. I use the amount of albumin rising to the surface as a gauge to see how quickly the salmon is cooking.  Remember, we are trying to go slow and slow, so you shouldn’t see a LOT of albumin, but you will always see some.   One way I address it is to baste the surface with honey.  The brush wipes the albumin away, and the honey quickly forms a protective seal. Another tip I tend to use if I’m serving this to guests is to sprinkle parsley on it, as it makes all the whiteness “go away” in their eyes.