Big problem with Mould

Maintenance

I have been Letting this ex-local Authority property for the past 15 years without any problems.
Six months ago mould started to appear in the (external) Bedroom and Lounge walls and very quickly almost covered them. I told the tenants to stop drying clothes and bought them a new washer drier and a dehumidifier.
The Council say that it is nothing to do with the building and suggested that we wash the walls with water and bleach.
However the problem has not gone away
I need to solve this problem so any suggestions would be appreciated.