icon behind applications app in taskbar on macbook pro

Check to see if that is the icon of the first listed app in your Applications folder - it shows that way in my dock (and I am guessing the taskbar is the dock?) If so, there is nothing you can do except possibly moving an empty folder into that spot (name it 0 0 so it shows up first).

I knew you meant the Dock. Sometimes a cleanup will realign icons and pull then up from the bottom. Was worth a try.

EDIT: AH! Found it! If you right-click the Applications icon in the Dock, you can change the appearance. Mine had what you and babowa described until I changed the "Display as..." option from "Stack" to "Folder":

Sometimes the "Clean Up" function will rearrange things and that might dislodge that offending icon. Make sure the menubar say you are in the FInder and you can find the clean-up command under the View menu: