After looking through all the available info for S5 icons I feel like
It is coming from something you have downloaded.
Try going to "Sounds and Notifications " and scroll down to the bottom to
"Application notifications". Open each application and turn on the "Block
all notifications" while watching to see if the icon goes away.
Just a thought... the icon you show, (from what I can see of it), looks just like the reminder icon on Windows 10, Cortana. Are you by any chance sync'd to Windows 10?