Mould in 1 room

Hi all I own a 4 bedroom house which is let to 4 friends. One of the tenants went travelling for a month with everything closed and since returning noticed mould on the wall. I've advised her to ventilate, use bleach to kill the mould. Its helped but she says if she keeps the door close for even a day a light smell of mould returns. We had someone come and check and he said there was no moisture coming through the walls, it's simply a ventilation issue. But it's the first time this has happened in the 7 years we've owned the place.

Whats the next step? Repaint with antimould paint?

Tenants are lovely and I have a good relationship with them.