efflorescence is the white stuff you see on new external brickwork after it rains, it goes eventually.
The hairy inside stuff is salt crystals growing from ground salts which have come into the walls and plaster from outside via the ground moisture. Its a sign of damp penetration. Even if you seal off the damp ingress by having a chemical dpc inserted, the salts will still be there and as they are hygroscopic they will attract atmospheric moisture and continue to grow, even through new paintwork. Moist areas like loos and bathrooms are the worst. We had our WC completely redecorated after this same problem including partial replastering but it has still come back because the insurers wouldnt pay to have a dpc inserted or to re-plaster the whole area.
1- find moisture source and eradicate
2-hack off plaster to 18" height
3-replaster in sand and cement with a salt retarding additive -builders merchants have it
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