@eypicasso I'm a newbie, too, so don't treat my remarks as gospel, but I've read on other comments that the "completed" mark refers to native publishing. I.e, it's finished it's run in Korea. (basically, think Naruto - completed in Japan, but still getting translated/released here.)
Once a title is *ACTUALLY* completed, you'll find a little blue [End] tag next to the last chapter (see Isekai onsen for example)
As for if you should add it to your watchlist? Go for it. If it ever gets picked up, you'll get it displayed in your follows tab!