Put that interface as child of that moving object? Or set its position to be the same as the object every frame, using a script. Also make sure that interface is within the same CanvasLayer (or none if the moving object isn't in one).
For resizing interface when window size changes, look for stretch modes in project settings. Also, make use of anchors, avoid setting static GUI sizes when possible. You may want to take a look at this doc https://docs.godotengine.org/en/3.1/tutorials/viewports/multiple_resolutions.html