This episode was originally titled "The Citadel". This was then changed to "Su'Kal" shortly before release.
Doug Jones (Saru) is seen without his Kelpien make up on.
Saru is a Kelpien. The 'visions' he experiences while listening to The Elder's lullaby are his own memories of his home planet, Kaminar and himself, at the moment he chose to leave it. Several of these shots were reused from The Brightest Star (2018), which tells the story of how he left home to join Starfleet.