Mold spores are everywhere. They occur naturally in our environment. Most every breath you take has some sort of mold spores in it. You have some on your body right now. It's no big deal.
Mold is the American way for spelling mould. The spores which cause mould are everywhere and we constantly breath them in. Our bodies are superb at negating any threat they present but that doesn't mean we should inhabit a room with very high concentrations of the buggers. When these spores get access to moisture they germinate (like any seeds) and the result is mould. Mould will flourish where there is damp and still air. Wipe the mould away with a damp cloth and then dry the area and keep it ventilated. I suspect the damp on the sheet covering the headboard is coming from your breath. Hoover it once a week and all will be fine - and stop worrying.
I sleep on a sofabed and have a bookcase which has a sheet nailed over it as a headboard, and one day I looked behind one of my pillows, which was against the bookcase and saw mildew/mold on the bookcase sheets. I am now replacing the bookcase and a lot of other things where I live, going into hundreds of pounds because I am very concerned that spores, microbes, whatever, from the mold has attached to me, my hair, my skin, and then I have touched other items and mold has transfered to those items. Is it possible for this to happen, for mold to transfer from that sheet behind me, to my hair, skin etc. ? Need knowledgeable answer, thanks.