how do I remove " ib.adnxs.com" pop-up from Safari version 9.1.3 ?

I'm on Safari version 9.1.3 looking at a website, when this window drops down repeatedly say, "The certificate for this website is invalid. You might be connecting to a website that is pretending to be "ib.adnxs.com", which could put your confidential informational risk. Would you like to connect to website anyway?" The window gives three choices; Show Certificate, Cancel, or Continue. I Cancel because I am not on that website, but it keeps dropping down. this happens on multiple websites. How can I get rid of it.