How do I remove what appears to be malware on my Nexus 5? I have a Nexus 5 phone running Android 6.0.1 (security patch level 1 Jun 2016).

It appears that over the past few days it has become infected with malware.

Periodically (every 10-15 minutes) a transparent notification appears near the bottom of the screen that looks like this (the numbers will change, and possibly the non-English text).



Here's what I have tried so far:


Installed Malwarebytes and ran a scan but it couldn't detect it.
Installed a few Antivirus programs (AVG, 360 Security) and it didn't find anything out of the ordinary.
Installed OS Monitor to see if there are any suspicious processes running or network connections open but nothing stands out.
Closed all running apps
Rebooted the phone


The notifications keep popping up.  I'm a bit stuck here - this definitely looks like malware, but not sure where to go from here.  Obviously a factory reset is an option, but I'd like to expore other options first.

Any recommendations about how to proceed?
Update

After 2 days, uninstalling ES File Explorer seemed to do the trick - I'm not seeing these pop-ups anymore.



After further research this appears to be caused by an update to ES File Explorer (see this question).  I have now uninstalled that app.