The Aladin stack shows all elements that have been loaded during your Aladin Desktop session. The stack is a pile of individual planes, which can be pointed images, HiPS surveys, catalogues, graphical overlays, filters, data cubes, etc...
Each time you load a new element, a new plane will be added to the stack, usually at the top. To understand what is displayed in the main Aladin panel, imagine you are looking through the stack, from the top :you see all elements overlaid on top of each other, until you reach an opaque plane (in this case the DSS HiPS survey).
Clicking on the trapezium icon at the left of a plane allows you to toggle it off, or back on ; this can reveal planes below it.
Clicking on the name of a plane makes it the active plane in the stack. It is highlighted, and buttons such as « Properties » will apply to this active plane.
If you want to delete a plane from the stack, make sure to select it, and click the delete button. You can select several planes by pressing the CTRL or Shift keys while selecting them, and delete them at once.