Android suddenly pops up ad
I advice you to try and find which applications are running by using a program that displays your current processes. One of those will be the adware. You could also try and install some android anti-adware blocker, or even cut out the banner with an image editing program, make an image search and find which program or advertisiment the image matches, then you will know which program it is and you can find instructions how to remove it. 

If you install termux you can run the command ps -al|more and paste the output, then we will see which processes are running.