White powder on brick wall.

He’s dealing with efflorescence or salt petering (know by others), and can be removed by substances like salt neutralisers. Anti-Sulphate Tanking & Surface Treatment - Long Lasting Protection Against Salts in Masonry might be something adequate.

Efflorescence is the classic issue here, usual areas such as gable ends of Propertys that take the brunt of the weather. We would advise a salt neutraliser and to weather protect the affected areas.

efflorescence / salt is usually caused by wet bricks caused by damp and poor ventilation in the cavity

It’s called efflorescence it’s when the salt comes out of the sand. People think it’s the brick but it’s not the case there are ways to prevent it like not building damp bricks keeping them frost free ect sometimes it can’t be evaded and it’s water dripping down a vent or downtown and this would lead to the salt coming out it can go away naturally but may take a very long time and it can ruin the look even though everything is Structurally sound that’s why it’s often forgot about on new builds but a way to get it or is to acid wash it and wire brush it not to hard as can take the face of the brick but gradually and remember to water the acid down as I think they always come to strong and can make it worse there are products out there like salt away but you kind of pay for what you get when buying the cheaper stuff

It will be efflorescence commonly known as salts. It's caused by water seeping through masonry or cement based material. Its very common. use a stiff brush or some smooth surfaces, you may be able to use a stiff brush. Rinse with water if you go into a good builders yard that deals with brickwork they will show you the best product to us.