The white substance is called albumin. It's a protein that is naturally present in fish. When exposed to heat, particularly during the smoking process, it is forced out of the muscle fibers and becomes visible as a white substance. It's normal to have a small amount surface; you just don't want too much to seep out. This can happen if the internal temperature of the smoker is too hot and the fish cooks too fast. It's not harmful at all, but excess albumin can result in your salmon being too dry. To avoid this, start by smoking the fish at 150°F and then slowly increase the temperature to no higher than 180°F during the smoking process.