Move expdp part file before job end
It is not advisable to move files before the export job is finished. The reason is that when exporting, the utility "builds and maintains the master table for the duration of the job. At the end of an export job, the content of the master table is written to a file in the dump file set." (see Oracle's documentation at the following address: https://docs.oracle.com/database/121/SUTIL/GUID-6BDC1CC8-8596-402D-B016-602985B97AB6.htm#SUTIL814) If you move a dump file before the job is done, you risk corrupting that process and making the job fail.

Is there a reason you need to move the files before the data pump job is finished? If not, simply wait for the job to complete. While being able to move files as soon as they are not in use would be very useful for large export/import jobs, you will save yourself a lot of time by letting the export finish before manipulating the dump files.