quit out of CCleaner's background process. You can kill off CCleaner in Activity Monitor by clicking the X button right above CPU time listing. Relaunch AppCleaner and proceed to uninstall CCleaner.
You can allow AppCleaner to clean already running apps if you go to AppCleaner Preferences via Command+, This is universal for most Mac apps - there might be one or two apps that doesn't conform to that. These are mostly Windows or Linux ports. You'll have to relaunch AppCleaner for its new cleaning preferences to take effect.