[Griffus]

Efflorescence is a white powder that results when mineral salts in mortar are dissolved by water. These deposits usually disappear after a few years.

nasty stuff   

[Kev Martin]

That is not really efflorescence per say it is Limescale caused by the overflow. It can be cleaned with Phosphoric acid you don't need brick acid. There is also a product made by Aqua Mix called Ex treme clean which is designed for this problem. It has an acid in a strong cleaner that is buffered but as a few have said already it will be so good when it's finished it will make the rest of it stand out like a sore thumb. My approach would therefore be to try and clean it dry with a rotary knotted wire brush on an angle grinder first.

Kev Martin
Tiling Logistics Ltd