The Ghost of Our Love side quest in Hogwarts Legacy is obtained in a different manner for each house during the Tell Professor Fig About the Map Chamber main quest. For Gryffindors, the Map with Floating Candles can be found on a stone coffin in the cemetery where Sir Nicholas takes you. For Hufflepuffs, you can loot the map from a chest in Upper Hogsfield.
Once you have the map, travel to the Forbidden Forest Floo Flame on the road between Hogwarts and Hogsmeade. You’ll then need to wait until night. Approach the stone bridge to the west (there will be a love letter on the end if you haven’t collected it yet) and cast Lumos.
This will cause candles to appear. Follow the candles to their destination causing a chest to appear. Open the chest to receive the Treasure-Seeker’s Scarf appearance completing the quest.