The general concept is that a landlord is supposed to provide a clean and healthy living space; and the tenant is not supposed to undo that. So, your issue turns on whose act or omission caused the mold. Right now, the landlord is claiming that you caused it. If she can prove it, then you may be on the hook for the remediation.
I would be interested to know whether there is mold in any of the other apartments with tenants who keep their windows closed and run the AC all day. If so, it would tend to support your claim that you did not cause the mold by leaving your windows open a lot.
Dennis Phillips, Esq.