Revisions log for Answer #1647

This fungus occurs only when the plaster dries, with a lack of ventilation. It occurs in almost all gypsum plasters. Maybe the corner was blocked up or there was no air flow. And since after plastering the walls, humidity increases many times, it is the best thing for the fungus. The 1st thing to do is to ventilate the room. The second is to burn out with a heat gun, so it does not spread. After the entire plaster has dried, knock down to a few centimeters and after priming, re-master this piece.