You can use a TextureRect instead of a Sprite and connect it's
mouse_exited-signals to show or hide the label respectively:
connect("mouse_entered", self, "_on_mouse_entered")
connect("mouse_exited", self, "_on_mouse_exited")
The script above assumes that your label-node is a direct child of the TextureRect, if that's not the case you need to adapt the path to the label-node accordingly.