Sometimes a chalky white layer can appear on exterior walls. Whilst some exterior paints are designed to do just this, sometimes it can be the sign of an issue.
There are a number of reasons Chalking can occur:
Water borne paints being applied in temperatures below 5 degrees.
An interior paint being used for an outdoor application
Inadequate sealing of a porous surface
Over-thinning a coating
Some exterior paints are designed to chalk as part of a self-cleaning mechanism, so make sure to check the product data sheet first.
Remove chalk by washing or power washing the surface with a surface appropriate cleaner.
For best results firstly ensure that the correct primer/base coat is being applied. Applying a Stabilising Solution or an Alkali Resisting Sealer before coating will help to reduce the risk of chalking.
Secondly, always check a products data sheet and back of pack to double check that it's the best product for your chosen surface.