Sorry landlord hat on again. There is lots of advice on the internet, but if damp is bad now it will only get worse in winter.
Keep doors open as much as you can, and bathroom windows, so a good supply of door stops especially for the bathroom. Don’t dry clothes in rooms that are prone to mould. Keep bathroom floor dry, so get bath mats which are then hung up. Dry off condensation that forms on windows.
Extreme, but you could buy a dehumidifier.
Mould is not healthy, so clean it. Purple Cilit Bang is good. My experience as a landlord is that I can go several tenancies without a problem, then it can be really bad. And that it is all about ventilation (unless the property has genuine damp problems- in which case the landlord needs to act.) stereotype- but it’s boys who never open windows or doors and whose rooms have a real boy odour.
DD and I cleaned up her flat. There was lots of mould in the kitchen, bathroom and smaller bedrooms. So far it has not reappeared, so hopefully it was poor habits of the previous tenants.