Your mold problem, is more than likely due to moisture that is leaching into the mortar joint of the wall. If it is like a powdery white looking substance. Painting over it with latex paint will do no good. The best solutions is to wash the wall with a strong solution of muratic acid, (following the instruction very carefully) then you can apply your primer and paint. Hopefully this should solve your problem.

The Yellow Rose
Don Cagle