I created a Circle of Dreams Druid for a campaign. DND Beyond added the spell Temporal Terrain to the character as always prepared. I cannot figure out where this spell is coming from. Searching for the spell on DND Beyond (or even Google for that matter) yields no results. Where is this from?

It's a homebrew spell, and it sounds like whoever made it made it 'available for' Circle of Dreams druids. The homebrew system is set up to think that 'available for' a subclass that uses prepared spells means that the spell is one of the subclass spells that they automatically get without having to prepare it, which is why it's automatically showing up for you. Someone in your campaign has that spell, as homebrew is automatically shared in campaigns. Ask the other people in that campaign, and see if they can either remove it from their collection or remove that functionality (if they're the ones who made it).