I've just tested this. Cleaning "System/Temporary Files" with BleachBit removed following files:
Delete 4,1kB /tmp/sni-qt_copyq_7487-uq6jv3/icons/hicolor/48x48/apps/copyq_7487_3b113d95128bb105a6dd1b89f6652e0a.png
Delete 4,1kB /tmp/sni-qt_copyq_7487-uq6jv3/icons/hicolor/16x16/apps/copyq_7487_3b113d95128bb105a6dd1b89f6652e0a.png
Delete 4,1kB /tmp/sni-qt_copyq_7487-uq6jv3/icons/hicolor/32x32/apps/copyq_7487_3b113d95128bb105a6dd1b89f6652e0a.png
Delete 4,1kB /tmp/sni-qt_copyq_7487-uq6jv3/icons/hicolor/22x22/apps/copyq_7487_3b113d95128bb105a6dd1b89f6652e0a.png
Delete 0 /tmp/unity_support_test.1
Disk space recovered: 16,4kB
Files deleted: 5
So it seems that Ubuntu wrapper library for displaying QSystemTrayIcon stores icon image file in temporary directory. Oddly the library will just use invalid icon if the image is removed.
After CopyQ is restarted icon is back in place.
Removing temporary files randomly is not a good practice. Yes, there could be some stale files, but it may also contain some important data or backups. CopyQ uses temporary files when you edit items in an external editor.