Speak with a landlord-tenant attorney: there are several options, but it is best to have legal help in pursuing them. All rental units come with what is known  as the "implied warranty" of habitability, or the obligation, imposed on the landlord by law, that he or she provide a rental unit which is "fit for its intended purpose" (residence) or habitable. Serious mold conditions, due to the health risk, can violate this warranty. Depending on the exact severity, circumstances, and whether or not the landlord has been provide notice (and when) of the condition, the tenant(s) may be entitled to:1) Treat the tenancy as terminated due to the landlord's violation of the implied warranty of habitability (i.e. that they were "constructively"--or effectively--evicted due to the lack of habitability) and move out early without penalty;2) To pay for remediation efforts themselves, then deduct the cost thereof from rent ("repair and deduct");3) To seek monetary compensatation (rent abatement) for the time they have been living with the impaired  condition.However, when the situation is not serious enough to violate the warranty, or the tenants do not exercise their rights properly,  they can end up liable to the landlord; this is why the assistance of a landlord-tenant attorney is strongly recommended.