What's the white liquidy substance that can appear when cooking salmon?
That white stuff is albumen, the same protein that makes the white of chicken eggs.  The albumen is part of the salmon's blood, which means that your fillet was fresh.  A trick for dealing with it was developed by Bruno Goussault while collaborating with chef Michel Richard from Citronelle in DC: soak the salmon pieces in an ice cold brine. The article in the New York Times that told the story recommends soaking the salmon fillets for ten minutes in a 10% brine (by weight).  To make the brine, dissolve three tablespoons of Kosher salt for every cup of ice cold water and make as many cups as you need.