Mould on internal walls (often called black spot mould) can be the result of moisture being drawn in through the walls of your property and results in your having to re-decorate on a regular basis.
If the damp patch on the wall is more than half way up the wall (in a downstairs room), chances are your suffering from penetrating damp.
There are several causes of penetrating damp, some of which are easily cured. Check to see if you have blocked gutters or drain pipes which may be making rainwater run down the wall of your property. If you have cracked or broken render or pebbledash, or if the front of the bricks have fallen off (or blown), this can be a major cause and needs immediate attention.
A new resin based wall coating may be the answer. If you already have render or pebbledash or some other coating on the outside of your property, you will need a specialist application. One such company, Wethertex UK offers free advice and free surveys. On their website is information and photographs relating to penetrating damp and details of their products which will permanently cure the problem.
The web site is found at http://www.wethertex.co.uk