Yes, pgsql_tmp, you ought to be looking for a sudden and drastic jump in space utilization around the time of the error message. You're not concerned with the current space utilization, but with the one around that time, because, it probably got freed right after the error was raised.
How many times has this happened ? what kind of queries were running at that time? can you identify something that could have required lots of temporary space?
2018-12-07 06:21:55.504 EST 10.140.181.89(35868) CDS CDSLBXW 13748 SELECT PostgreSQL JDBC Driver 53100 ERROR: could not write to tuplestore temporary file: No space left on device
I see this in the pg_log file, but #1 can't figure out what "tuplestore" is (Google doesn't help), and #2 there's lots of space on all my file systems. data/base, where pgsql_tmp lives, has 96GB free.)