you see the those torches behind you, there are a few of them in the room you can light to unlock the door.
Ah, I guess I was supposed to figure that out in either this room or the first room where the tutorial teaches you how to lewd a slime (amazing first choice btw)
I guess my issue was that both with that room and this one you can just walk right through the pillars blocking the door and into the next room so I never had to "Figure that out".
I'd suggest putting a little hint in the tutorial about lewding the slime something like "some objects can also be activated by charging this spell as well"