Mold on walls after tenant moves out due to poor ventilation?

Oh dear.... someone please define "adequate ventilation" and then it is still an interference with fundamental rights, which every judge tears apart in the air.

I understand the problem well, but I repeatedly observe how rights and duties stand.

A sentence like "The tenant must ventilate adequately" would regularly be void.

I say this without any legal background.

Yep, I wouldn’t necessarily get into an argument about that either. If you’re sure that the mold only appeared due to lack of ventilation, I would follow nordlys's advice and then paint over it. A bit of superficial mold doesn’t have to become a problem. However, to me, it looks more like a typical cold outside corner. Water tends to condense there easily. Is the apartment properly insulated? Maybe check with a laser thermometer or even a thermal imaging camera.