A word for 'independent episodes'

These are stand alone episodes, as they "stand alone" from the main story arc.

TV Tropes goes into detail, and provides multiple examples:

An episode that can stand alone on its own with a self-contained story that does not need prior viewing of any other episode to understand. It's usually an episode that breaks from the current arc to focus on a one-shot subplot or character; for example, if the arc is about stopping The End of the World as We Know It, a Stand Alone Episode can be about dealing with a Sealed Evil in a Can that's released at the beginning of the episode and resealed by the end of the episode, never to be mentioned again.

A Stand Alone Episode can also be a Beach Episode, Breather Episode, or A Day in the Limelight, but not always; the only prerequisite of one is not to follow a script that goes on for more than one episode. When such an episode happens to be a Season Finale, it is a Dénouement Episode.

Many shows can be considered a long run of Stand Alone Episodes. Comedy series usually consist entirely of Stand-Alones because each episode usually focuses on a different gag or zany schemes. Likewise, Adventure Towns series generally consist of Stand Alone Episodes.

In arc-heavy series, a good Stand Alone Episode can be the hook a die-hard fan of the show can use to pull others in, due to its self-contained nature. Likewise, even fans of the arcs will often cite a Stand-Alone as their favorite episode; an arc episode is difficult to separate and appreciate outside of the arc which contains it, but a Stand Alone Episode can be fully appreciated of itself. Sometimes, though, writers will want to revisit the plot of a Stand Alone Episode and create a later episode that expands on the earlier story; this is a Sequel Episode.

By their very nature, the pilot episode for a show is usually a Stand Alone Episode. Compare Filler, although the label is usually only used when a stand-alone episode isn't really good enough to stand at all.

The proper word for the same is perhaps “Episodic”

OED

a. Of or pertaining to, or of the nature of, an episode; incidental, occasional.

1856 D. Masson Ess. Biogr. & Crit. 257 Such incidents as these, episodic as they were to the two great topics of Wilkes and the Constitution and the growing disaffection of the American colonies.

1879 ‘G. Eliot’ Theophrastus Such vi. 123 His episodic show of regard.