There must be moisture somewhere in the room -- maybe humidity levels are too high, maybe there's a plumbing leak, maybe windows weren't installed correctly -- lots of possibilities. Definitely keep running the dehumidifier but I would recommend calling someone to determine and fix the root cause. Also make sure you're using something that actually kills mold (like vinegar), not just disinfecting, or the mold is likely to come back. I agree with replacing the mattress too. Sorry you're dealing with this, we had a mold problem recently too and it sucked 😞
It’s so annoying as it’s the only room to have it! Luckily it was just the mattress protector rather than the actual mattress so I assume I caught it early. Only 18 months of living in the house so clearly just finding issues still 🤦🏻♀️
I would also be replacing the mattress. Mould spores can be tiny so not sure I'd be comfortable leaving it. UK is pretty humid really. Not jungle humid but if you went to somewhere like Norway they have really 'dry' cold and think you'd notice a big difference. Hope you get it sorted! Not sure what to suggest other than a dehumidifier and making sure you're regularly letting the room breathe.
I'm glad you caught it early! It can spread like wildfire if left unattended in the right conditions. Ours was only in my son's room too 😭 we went nuclear. Just know mold issues are EXTREMELY common and you likely didn't do anything to cause this.
There must be damp in the room. To be honest if there’s mould on the mattress I would replace ASAP rather than just flip it.