That one was alright. With Charles from V as the Harlequin Space Ghost. It
was very derived. The basic idea had been done to death by Trek at that point. Then DS9, V'Ger & even Beckett's Folly
I think, repeated the basic theme which is:
Malavolent non-corporeal alien energy being terrorizes residents of Trekverse. Then Captain Adventure & crew figure out what the baddy alien is via an overkill of Treknobabble & pwn it's arse.
"Wolf in the Fold", "Day of the Dove", plus
many other episodes from all the Trex, covered the same ground as "Sub Rosa".
The TOS episode with Zeffy Cochrane was an exception as that bodyless entity wasn't a baddy