The title of the site itself is “Stella Prisma Teaser Site” in Japanese, so it’s definitely teasing something new. It comes with a video titled “New Game’s Voice and Sound” and described as “first public voices and sounds for a new game” if you needed any further confirmation.
Some will probably think about Stella Deus, but the connection is most probably unfitting, as Stella Deus wasn’t by Square Enix, but by Atlus. Japanese developers just love to use Italian words in their titles, as “Stella Prisma” means “Star Prism” in Italian or Latin, even if the translation ignores grammar as usual.
Below you can see (actually just hear) the teaser video. No other details, including platforms, genre and release window, were shared for the moment.