Common problem, looks like rising damp, older properties didn't have DPM under the back hearth slab. Fires used to keep it dry/down. Have to sort it out from the electric fire side - new conc back hearth with DPM/DPC underneath.
In theory it might be roof flashing failure but that usually shows in the walls not on the ground.
In theory it might be roof flashing failure but that usually shows in the walls not on the ground.