So, like when are you getting these alerts? Or is there an app that you are using that is showing this? This is most likely from a rogue app that was recently installed or from a website.
As stated, clear the cache and data from your browser, reboot and see if it still happens.
Moving forward, I would also install an ad blocker from the play store or something like Adguard, from their website, adguard.com and install it from there. I use Adguard and never any issues such as this.