I had something like that on my old phone while browsing reddit on the Android subforum (mine said something like your LG D802 has a virus or something). I simply pressed backspace, but there was definitely no sign of anything ominous going on in my phone after that. I must have run into some dodgy adaware there.
Personally I wouldn't worry too much about it if you didn't click on the "ok" button, but it's always best to check and run a few virus checkers on your phone. If you're feeling paranoid, backup anything important on your phone and do a factory reset.