SQL Server 2016 Job running forever
Sounds like a bug. Try scripting from the GUI directly and see if your syntax is wrong somewhere.
Since the problem still occurs when running the script by itself, it doesn't have anything to do with the fact that it is running from a job. My guess is that the EMC application isn't properly reporting back to the dbo.emc_run_backup stored procedure that is has finished.
If that's the case, I'll bet scripting to CLI (ddbmsqlsv.exe) instead of T-SQL might behave differently. (You would need to change the job step to CmdExec instead of T-SQL.)
This may be a version problem, from what I can tell DDBoost didn't officially add support for SQL 2016 until version 3.5:

Changes in the Microsoft application agent 3.5
The following list provides the changes in the Microsoft application agent 3.5:

Added support for SQL Server 2016


Can't find a public link, but this is from the EMC® Data Domain® Boost for Enterprise Applications and EMC ProtectPoint™ Microsoft Application Agent Release Notes for Release 3.5.x.x (EMC document #81497).
Even if it worked before, a recent update of SQL 2016 may have broken it.
So my recommendation would be to upgrade to 3.5.0.1 or 4.0.
If you are unable to do that, all I can recommend is opening a support case with EMC.