In KDE4, the system tray icon changed color. Now, in KDE5, the icon just slightly animates, and it's far too subtle to notice. I really wish there were an option to make it turn a bright color. Reproducible: Always
It is probably a icon theming issue. Adding VDG for inspection.
I just tested with Oxygen instead of Breeze (I prefer Breeze for reference), and the issue was the same. The icon was different, but it just sorta bulges in a pulsing way with no color, and since the system tray icon is pretty small, I don't really see it unless I actively give attention to the system tray.