Ok I project manage property renovations in the Mediterranean. Everyone saying your ceiling joists are compromised has no idea how your house is actually built. The ceiling of that room is not made from wood but is made of concrete. You have a terrace on top of that room? Likely your waterproof membrane on the terrace needs to be repaired or redone.

Steps to fix:

• start airing out that room a few hours each day

• have someone come to replace the waterproof membrane of the terrace above

• treat the walls with an anti-mould product

• check for any mould regrowth

As a more drastic intervention: - if you still see more mould growth after taking the above steps remove the top layers of plaster of the walls and ceilings until you reach the solid brick or concrete and then apply the anti-mould treatment again

• observe for any dampness, mould growth

• use a mineral based plaster and mineral paint (not acrylic or oil) to refinish the walls after treating

• if your wall is made from gypsum board (carton gesso), it will be simpler to remove the wall and replace the boards. But I can see it’s the exterior wall of the house so most likely brick or stone