Mold usually arises from excessive moisture and is the responsibility of the landlord. A tenant is generally no more responsible for mold remediation than would be the case if there was a leak in the roof unless the lease was quite clear that the maintenance obligation included such major repairs.
You have to demonstrate two things (1) that the mold arose because of something the tenant did that created an unusual level of moisture leading to mold, and (2) normal maintenance of the type one normally expects of a tenant would have prevented the mold. Unless you can prove one of these two things, you as owner are responsible for the mold remediation.
I would have advised you to do the remediation as soon as the mold was discovered and then look to hold the tenant responsible for the cost. The longer you wait the higher the cost. If the tenant is responsible and can pay you recover your cost, if the tenant is responsible and can't pay, you still have to get the mold out of the house otherwise you can't rent it. If you are found responsible you will have cut your losses.
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