It is not good to be dismissive of mould as it can have very nasty effects, on people and buildings.
The circumstances where these tenants have taken action against 2 previous landlords, and now threatening a third, when the current premises are new, makes me wonder if it is their possessions which have been contaminated in the first mouldy premises and they are taking the problem with them from home to home?
If there has been water ingress into a room and you are negotiating remediation with the builder, it would not be a bad idea to consult a mould expert to ensure appropriate work is done by the builder to stop future water ingress and any water damaged materials and mould (hidden as well as visible) are removed. Mould is often part of a building defect claim, with the cost of the expert included in the claim, and you should obtain formal advice before the defects liability period expires.
The usual medical advice to a mould sensitive person is to remove themselves from the troublesome environment, and your offer to let them break the lease without cost should be attractive to your tenants in the current climate where rental property availability is up, but rental prices are falling.