Cheers, I actually sprayed a bad leak from the boiler in the loft with brick acid and did not rinse quickly enough and it's burned the light colour harling a shade of light brown.
Brick acid is intended to clean bricks, and specifically to remove mortar/cement marks from bricks by dissolving the cement.
Flinging brick acid directly onto mortar or similar substances will destroy it. You can't clean it off, as there is nothing to remove.