nothing very bad.but mold is some kingd of mashrooms and not exactly good to live in their environment.so keep yr rooms aired and dry them as much as possible and rent another apartment.when whe walls were covered with whatever they wer covered with,u were in no bettr environment.the presence of mold in the room or wherever means the place is wet and not aired.but not too bad for u.just bad.
dont be afraid just do something about it.if u cant move elsewhere then air and dry this way the rooms as much as possible.mold cant live without moist and dark.remember where mushrooms for eating are found.in dark wet places in the woods.so yr room is like this place for them so they thrive in them
