When you return to the Hotel Sedgewick you will have to turn on the generators to get power to the elevators. After doing this you’ll come back through the main lobby. Just double back towards the doors that would lead to the street and you’ll find a small coat closet off to the left(if you are facing the street). Inside this closet is not only a cursed artifact(so use your PKE to help find the room) but also a boatload of black slime all over the walls and ceiling. Use your slime blower to clear it all off and the trophy will unlock. It took a few coats of slime for me to get all the spots.