Mold in apartment

I just moved out of an apartment I had been living in with my husband for 3 years. We have a 6 month old son who shared a room with us while living in the apartment. Upon move out we discovered mold behind our dresser. This was a bedroom with no water source, the wall was exterior, but never showed signs of being damp (like water running down the walls etc.). I'm worried about any health risks caused by living with the mold, and wondering if anyone had experienced mold like this and what caused it. We're trying to figure out if the landlord should be held responsible since it seems the mold is coming from within the walls and could have been caused by a water leak outside the building.