Comment # 1 on bug 977040 from Wolfgang Bauer

Created attachment 674412 [details]
screenshot from an up-to-date Leap 42.1 VM, where it works

Works fine here, see attached screenshot.

Make sure your system is fully up-to-date.

This was indeed a "problem" in Plasma 5.4 as originally shipped with Leap 42.1
(IIRC), the taskmanager tried to lookup the icon in the application menu and
fell back to the generic executable icon (i.e. the green "play" button you
mentioned) if it couldn't find the corresponding entry for some reason (and
this did happen with YaST in particular).
There was an option in its settings to disable that though and always use the
window icon.

It has been improved in 5.5 which we have released as official update for Leap
42.1, it should not use the generic fallback icon now.