Something like this here?
#include <GUIConstantsEx.au3>
#include <WindowsConstants.au3>
#include <WinAPIGdi.au3>
Global $hGUI = GUICreate("Ghost", 100, 50, @DesktopWidth - 100, 0, $WS_POPUP, $WS_EX_TOPMOST)
GUISetBkColor(0x808080)
Global $iLbl = GUICtrlCreateLabel("Hover me :-)", 10, 10)
GUICtrlSetColor(-1, 0xFFFFFF)
GUISetState()
AdlibRegister("HoverChk", 50)
Do
Until GUIGetMsg() = -3
AdlibUnRegister("HoverChk")
GUIDelete()
Func HoverChk()
Local $iFadeIn = 8, $iFadeOut = 64
Local Static $iTransparency = 255
Local $aPos = WinGetPos($hGUI)
If _WinAPI_PtInRectEx(MouseGetPos(0), MouseGetPos(1), $aPos[0], $aPos[1], $aPos[2] + $aPos[0] + 1, $aPos[3] + $aPos[1] + 1) Then
If $iTransparency >= 0 Then
$iTransparency -= $iFadeOut
$iTransparency = $iTransparency < 0 ? 0 : $iTransparency
WinSetTrans($hGUI, "", $iTransparency)
EndIf
Else
If $iTransparency <= 255 Then
$iTransparency += $iFadeIn
$iTransparency = $iTransparency > 255 ? 255 : $iTransparency
WinSetTrans($hGUI, "", $iTransparency)
EndIf
EndIf
EndFunc