If he doesn't repair, and you want to stay there, you can usually repair (you spend the $ on repairs) and deduct from the rent. Keep a copy of all bills and correspondence with your repair person.
If you have any additional questions and it gets to the point where you want to move out, you should contact a local landlord-tenant attorney. Most L&T attorneys are less expensive than other types of lawyers, generally speaking.
There may also be a Department of Safety and Professional Services in your area if there is no building inspector.
You have the right to a habitable apartment. The landlord cannot just do nothing.
I see you contacted Environmental health -- have they confirmed that it's mold? You should not try to clean it yourself because if it turns out to be black mold, then you're breathing that in and that's dangerous. Contact a professional if the landlord doesn't repair. Give him a reasonable time to make repairs, but since this is an urgent problem, reasonable in this situation would be somewhere in the range of 1 week. That is more than enough time to give him to make repairs.
If you are in Milwaukee, you will need to discuss this with an L&Tenant attorney because there are specific procedures to withhold rent. In other areas of the state, withholding rent procedures are not crystal clear -- the statutes are not clear and it's open to interpretation. If you're going to withhhold rent, see if you can get a free consultation with a L&T attorney (it won't cost you anything but your time) and ask about withholding rent in the event the landlord doesn't repair.
I hope this helps and clarifies. If you could, I'd appreciate it if you can rate me when finished.
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I wish you the best of luck. If you need to follow-up with any questions related to this topic, you can send me a follow-up email.
Ronna DeLoe, Esq.