This looks like a graphical glitch probably caused by the graphics driver.
You could try switching the OpenGL compositor to 2 or 3 depending on what you have set right now. If both have this glitch you can also try XRender which will solely software for 3D effects on the desktop.
If you know how to update the kernel you can also try that. Debian has some more backports kernel available that you can install with the linux-image-amd64 package. You need to reboot to use the newer kernel. Newer kernels have improved graphic drivers that might help with this issue.