Has my iMAC been hacked?

When I use my iMAC on one desktop I only see the programs that I'm running.

But when I show the overview of all desktops I see on my desktop one in the right corner always a small white screen, which normally indicates some application to be running. (see enclosed picture)

When I go into the desktop I don't see it.

has my pc been compromised? And how can I check what causes this white screen? Any tips are greatly appreciated.