When cooking any type of meat, use meat thermometer to check if it's done. It should read 145 degrees F. If you don't have one, cut into the thickest part of the fillet. If it starts to flake, it's done. If your salmon is covered in white substance, it's overcooked. It's called albumin and it's a solidified protein. It doesn't affect the flavor and it's totally fine to eat, just doesn't look appealing.