How can I stop cleaner app from taking over when I turn on my flashlight

Flashlight app? What flashlight app? Any flashlight app was added.

To access the built-in light, swipe down from the upper right corner of the display to open the control center, and tap 🔦 to turn on the flashlight from there.

… Access Control Center on your iPhone - Apple Support

… Edit Control Center on your iPhone, iPad, and iPod touch - Apple Support

I’d wonder whether that flashlight app is the cleaner app.

Remove the Flashlight app. Sounds like this is a 3rd party app downloaded from the App Store that is bringing up an ad on the screen. You do not need an app in order to use the built in light on your phone and the light can be turned on in the Control Center.