Can someone please tell me what app is causing this pop up every time I open my phone? It's annoying, because I have to click on it every time just to use my phone. I've attached a screenshot.
At first check if you have installed any application that was installed by you or not. You can see the from setting >Apps (No rush if you are not much familiar with system apps and can leave them alone)
If you can't see any suspicious application, you should be cautious and maybe it is highly exploited. in this case you have to make full wipe and install a new rom.
if you can see some application that was installed by you but not from playstore uninstall that too cause Google developers have to follow their market and community standard .