Technically, the mold problem is the responsibility of the landlord..... The de-humidifier is your expense unless it is permanently installed in your apartment. Anything that a renter does (such as installations of appliances/fixtures, painting, major landscaping) which improves the value of the property is supposed to be reimbursed by the landlord. I'm only speaking from personal experience as a former landlord in Oregon. Things may be different in your neck of the woods.
I will say, though, that in ANY state, if a renter shows there's a health risk with their lodging, the renter has the upper hand.
Good luck
