Try defining the component in-line instead. You also don't want to use the ToolTip component because you still need to display the header text. Instead, just use a Label component and set the toolTip string for the label like so...
<mx:DataGridColumn width="26" textAlign="center" editable="false"> <mx:headerRenderer> <mx:Component> <mx:Label text="M" toolTip="My Header Tooltip"/> </mx:Component> </mx:headerRenderer> <mx:itemRenderer> <mx:Component> <mx:HBox horizontalAlign="center" click="outerDocument.moveToActive();"> <mx:Image height="16" width="16" source="@Embed('/images/backwardicon.png')" toolTip="Click to make 'Active'" > </mx:Image> </mx:HBox> </mx:Component> </mx:itemRenderer> </mx:DataGridColumn>