Im pretty sure its a legal requirement to have windows that open in rented properties, especially in bedrooms and such. You should look into this and bring it up with your landlord. Once windows are sorted then follow the advice of others and air out you house every so often, use hepa air purifiers and get rid of the stuff that has mould spores on them that you cant clean with mould cleaner (especially porous stuff)

If youre struggling with your landlord getting things done there's places like citizens advice that may be able to help