Read your lease agreement. Is there a clause regarding pets? Does it specify domestic or exotic pets? Does it mention size restrictions or anything that would indicate it is referring to dogs and cats? If so, you can make an argument that your snakes don't fall under that clause. There are numerous leases that would allow pets like fish because they are not considered "pets" under their definition, so it depends on what it says and how they define what makes a pet, a pet.

If exotics are listed not allowed under any curcumstances at the time you renewed or sign off on your lease, mold or no mold, you can't keep them.

If exotics are included as pets but there is a monthly or mandatory fee, you may still be able to keep them by agreeing to pay said fees or leave with your animals.

The burden of proof will probably fall on them if they insist your enclosures are the cause or encourage mold growth. I would not so sure to say that enclosures alone have nothing to do with it. They could try to argue it is not just the enclosures, but the maintenance and equipment that is causing or making the mold issue worse.

If you wish to stay, then hire a lawyer. They might back down rather quickly and leave you alone, at least temporarily, until they find something else to bother you with.

Depending on how long you have stayed, some states have a law where the landlord must give you a specific time period to leave that could extend to more than 30 days from the date of notice. Check your state's law and know your rights.