The pipe above this vent was dripping against the wall, the drops has been fixed and the wall has dried out. But the plaster/wall around it is an odd combination of powery and spongy, and feel like some special form of plaster was used, and the vent to the outside is coming away slightly.
Is there any simple, relatively neat solution to fixing this? Such as expanding foam. Probably needs to be semi weather-resistant as cold and wind come through the vent a lot.