Grey bar appears when installed plank dock

I installed Ubuntu 16.04 and installed XFCE desktop environment right away. And then installed numix theme and icon pack. Then I installed plank. After installation a grey bar appears. When plank is closed it dissappears.

Here's an image: Plank placement: Bottom

Plank placement: Left


Solution 1:

Finally, I figured out the answer for my own question.


Steps:
  • Open "Window Manager Tweaks" --> "Compositor" tab
  • Uncheck "Show shadows under dock windows"

    (I'm using XFCE desktop environment.)