Thanks for letting me know. This pop up is aggressive AntiVirus software advertising. I guess you use a Mac too? I looked into it and it definitely isn't hidden in my site, but I cannot find how they connect it to my pages.
MacKeeper, the company behind it seems to be known for this method. All you have to do is click it all away and open the page (my page or any other where it happens) again. All should be fine. It reappears when you delete the cookies and your browsing history.
Anyway, thanks for telling me!