Why does the CCleaner tray icon come back after restarting my PC?

This is because the CCleaner tray icon displays differently in CCleaner v5.46 and beyond: 

You should see it in the Windows notifications area if the Smart Cleaning process is running. To stop this, switch off Smart Cleaning by unticking the checkboxes in the Smart Cleaning menu.

If you still use v5.46 or an older version of CCleaner:

We highly recommend you update to the latest version.

Doing so gives you access to the latest and greatest features and can help improve your cybersecurity.