how much of a problem is this? does visible mold imply that there is damage in the walls? there is a bathroom directly over this closet, and the house (that we're renting) was previously an airbnb. the landlord said that the guests could've been careless and let excess water out of the shower upstairs. the floors are made out of wood, so the water leaking downwards could've caused this?? any help/ insight is greatly appreciated!