According to Refinery29, the white stuff in salmon is a protein called albumin that generally comes out of salmon when cooked at higher temperatures. It is a sign that your fish may be cooked or overcooked so keep an eye on your fish as you cook it. The protein is not harmful and can just be wiped away. If you have extra time you can try to brine the salmon as Cook's Illustrated describes in their article about tricks for avoiding salmon's "white stuff."