Black mould behind bed

This is because you need to ventilate the property. I’ll bet the wall is an external wall too. What’s more, the previous issue was due most likely to the condensation too.

Black spot mould is an atmospheric condition and is not associated with dampness but with poor air exchange and a relative humidity in excess of 65%. The moisture is coming from sleeping in the bed . Clean the wall with white vinegar to kill it. If the room is very small you probably can’t get a good air circulation behind the bed so just repeat the treatment as needed. Check your mattress for mould too, this you can’t clean unfortunately.

Water can sometimes penetrate exterior walls if pointing is in need of repair or render cracked. More usual mould appears because of lack of ventilation & or lack of insulation making the wall cold, warm air & steam condense forming mould. If insulation is the problem it can be resolved externally , in the cavity [ if it has one ] or internally using insulated plasterboard.

It is more than likely that it is caused by condensation if that is porblem you my need to fit a elite fan ot the out side wall very quied it take all the condensation out and lets fresh air very cheap to run john