These are salts which have come to the surface because of damp. The damp is probably coming from a hidden lead gutter and has penetrated the brickwork and migrated to the areas which are white and the surrounding areas. All timber joists and wall plates will be subject to wet and dry rot and become more susceptible to insect attack. All plaster will be infected by the salts.
Party Wall Surveyor serving Thanet and London
Third Surveyor serving England and Wales