I'm hesitant about this because I don't want to mess anything up with Steam.
You can right-click upon that cleaner within CCleaner and choose Analyze, then it will display a list of things that it can delete.
One caveat is in the past going back many years (I don't know if it still does) it has caused serious issues with Steam, and Steam's VAC, etc., see this search and then make an informed decision:
https://duckduckgo.com/?q=ccleaner+breaks+damages+steam
What I'd recommend is make a system disk image backup before attempting to clean it, better safe than sorry!