That is the Menu Bar.
Basic Mac OS X Graphical User Interface (GUI) Elements
clickable for bigger picture
Since that behaviour indicates, that the app is in your Menu Bar, click on the icon in the Menu Bar the arrow points at, since that note window will probably originate there. If a primary click (one finger click on the trackpad or left click on the mouse if you use your mouse in your right hand) does not work, try a secondary click (two finger click on the trackpad or right mouse button click) and see what it says.