House Flipper 2: Hidden Stain in Peacey’s Family House

The stain in question is located in the floor under the first step.
If you remove the bit of floor in front of the step you will see the textures z-fighting like in the image provided.
This is the area with the stain, clean this area well and the replace the floor you destroyed.
You can re-tile the floor with the free tiles from the unfinished room upstairs.