The page at https://secure.img-cdn.mediaplex.com says:

[Voxboy]

Anyone know what this is. It has started to pop up on my screen the last couple days and seems to be little info on the web.

 

[PolygonWRLD]

anymore info? what appears on the webpage when it pops up? have you downloaded any software from some less than trustworthy sites?

 

[david15kramer]

same thing happening to me..happens when i go to youtube and click on a video. only on safari i think though

 

[Voxboy]

Yes. YouTube as well. I run Chrome so....Nothing appears on the webpage other than the pop up. Any software I have downloaded is legit. I will try and do a screen capture

 

[Voxboy]

 

[david15kramer]

Yes. YouTube as well. I run Chrome so....Nothing appears on the webpage other than the pop up. Any software I have downloaded is legit. I will try and do a screen capture

same here and i don't do any sort of torrenting at all very strange

 

[PolygonWRLD]

Yes. YouTube as well. I run Chrome so....Nothing appears on the webpage other than the pop up. Any software I have downloaded is legit. I will try and do a screen capture

is it only on chrome this is happening? what websites is it happening? from some searches it appears to be some sort of advertising company possibly trying to use a popup to display ads here's a page on it http://www.helpfixvirus.net/how-to-completely-remove-img-cdn-mediaplex-com-pop-up-removal-guide/

 

[david15kramer]

is it only on chrome this is happening? what websites is it happening? from some searches it appears to be some sort of advertising company possibly trying to use a popup to display ads here's a page on it http://www.helpfixvirus.net/how-to-completely-remove-img-cdn-mediaplex-com-pop-up-removal-guide/

that antivirus isn't supported for OS X

 

[PolygonWRLD]

that antivirus isn't supported for OS X

you could attempt by doing it the manual way and by resetting your browser.

 

[DeltaMac]

Try downloading and running Malwarebytes Anti-Malware for Mac.
That should clear that adware for you.