We have a brick built shed which has developed a white 'Talcum Powder' mold. Is this dangerous ?

It's just salts coming out of the brick material and is nothing to worry about. It's not dangerous. It'll disappear over time with weathering but if you want to speed this up just brush it off. Don't use acid or for that matter any liquids, all that will do is dissolve the salt particles and force them back into the pores of the brick, from which they will re-emerge as it dries off.