applications - Why does this ad keep showing up below my notification bar and how do I stop it?

I keep getting an ad that shows up BELOW my notification bar. It is NOT in the notifications, nor on the homescreen, as far as I can tell.

I am using CyanogenMod 12.1 nightlies on a HTC M8. Screenshots below.

How can I get rid of this thing? It just showed up one day. I don't remember installing anything new (although I have autoupdate on for apps, and some update very often) right before it started. I tried ad scanners, but they haven't been much help.

I would rather not factory reset.

I can close the ad by clicking the small "x". If I click on the info button in the lower left corner, I can give feedback on the specific ad and ask not to see it again. If I click on opt out, it asks for my IMEI, which I'm not really comfortable providing.

The ads are all in Hungarian and Romanian. I live in Romania, close to the Hungarian border, so I'm guessing that is why.

_{(Click image to enlarge)}

Opt out page:

Answer

There are many complaints about the Premium Wallpapers HD app, which apparently started to show ads looking like this after its latest update.

In general, an Android app can draw anything over other apps if it has the “Draw over other apps” permission, so you can check which of your installed apps have this permission.