Trub generally doesn't float. You can skim floaties off with a slotted or mesh spoon. The whole point of a racking arm is to leave trub at the bottom of the kettle where it sits. You start with a rotating racking arm pointing upwards and then rotate it downwards when you are draining to the fermenter until you start to pickup trub. You are clearly leaving a lot of viable wort behind in your kettle from the picture you shared, at least 2 gallons. Trub is usually less than a quart left behind including all of the hops you use in the boil. I generally use a 400-mesh bag to filter my wort into the fermenter and the trub forms a nice damp ball in the bag.