This is a not-so-common situation but most hosts would definitely encounter for a small amount of times in the hosting career. Problem is that the guest is hyper-sensitive to some allergens that is present in the listing while most other guests don't have issue with it at all. In this case no party has any wrongdoing it's just unfortunate coincidence. Similar examples include but not limited to pet hair allergy caused by some leftovers of previous pet/service animal. Even if hosts have no pet policy it does not mean the listing has never accommodated any animal due to the presence of service animals and ESAs, let alone guests who knowingly violate the rule.
Usually my solution is that if the guest contacts me shortly after check in for allergy (which is usually the case), I would offer the guest a full refund, in exchange of the guest check out immediately and cancel the reservation (so that i can still get bookings from other guests). If it's too late I might charge for the first night but it depends on the situation. I also advise the guest to contact Airbnb to find alternative lodging. My bottom line is: Any night or partial night that the guest stayed must be charged and if guest used our property so cleaning is needed, then cleaning fee need to be charged. At most refund the nights the guest did not stay and if so, the guest has to check out before the specified check out date.
BTW regarding the mold, if you are not sure, conduct a mold test either by buying a mold test kit and send to the lab, or ask a professional to do so. That way you will have an idea if your property actually has mold problem or not, and it serves as an evidence against the guest's claim.