iMac 10,1 - Mission control trouble - GPU?

[Andrea Filippini]

Hi everyone,
when I use mission control (F3) to check apps, the window of Microsoft Outlook has some trouble (see screenshot).
Is it a GPU issue?
My specs:

macOS High Sierra 10.13.6
iMac (21.5-inch, Late 2009)
3,33 GHz Intel Core 2 Duo
8 GB 1067 MHz DDR3
ATI Radeon HD 4670 256 MB
Maxtor Z1 SSD 960GB

Best regards
Andrea Filippini