Probably some app you installed that begins with the letter "D". Check your running and downloaded apps to see if you can find something with a similar icon.
What recent changes have you made to the phone? Have you taken Nahoku's advise and checked what apps you having running in the background? When you swipe down the notification bar, are there any alerts you can dismiss?
That definitely may be it. Could you (or anyone with a Galaxy S5 or S6) just download the app and run it in the background and see if that same logo pops up (screenshot whatever)?
I am almost positive that's it but now I can't replicate it...???!!!! So annoying. And want to be 100% sure.