Only to whoever hired them! As general contractors, they are not obliged to report anything regarding mold in your home to anybody except you, as it is you who has to contact an environmental health professional for instance, in order to fix the problem.
If you’re asking this question because there is untreated mold in your house but you still want some work done, it’s best to get that treated first. Most contractors won’t work in a house that has mold present, for the sake of their own career.
This is because, in recent years, there has been a trend in clients reporting their contractors as responsible for the appearance of mold in their home, in order to pay for the damage by claiming against their insurance.
It is usually the case that if a general contractor spots some mold in your home, or you make them aware of it, they will advise you to hire someone who is specialized in remediation to take care of the issue before they start work.
That being said, if you haven’t told them about the mold and they discover it before you can let them know, they may be inclined to call a professional on your behalf, especially if they believe that they, or you, are in danger of inhaling deadly spores.
When you’ve hired a contractor to do work on your house, but you’re worried they’re going to report you to some kind of mold authority, don’t freak out. It’s not like there’s mold police just waiting to come and arrest you for environmental crimes.
However, they do have a duty of care to point out anything that they spot to you, as the homeowner. If they don’t make a point of letting you know straight away when it is seen (unless they physically cannot see it) then you probably don’t want them working on your house.
A good contractor knows just how damaging mold can be, and will immediately stop working on the property (or should!) in order to have some remediation performed first. If they don’t, then you’re working with a shifty person with something to hide.
Perhaps they’re not fully qualified, or maybe they aren’t very good at their job, but I certainly would not entrust my family home to a contractor who sees mold and doesn’t immediately do something about it.