remember this old formula.
2 pails 2.5 gal water in each.
to 1 pail add 5 lbs NaOH, stir.
the other a box of corn starch, stir.
pour starch water into HaOH water,stir.
Thick and gloppy.
ALL SAFTY GEAR ON AND USED, STAY ALIVE.
caustic paste will do it.
problem is its gotta dwell, caustic on brick equals effloresence.
or
chisel off all you can, try to get it smoothed to a residual smear, then get a diamond grit sanding pad and wet buff the smear out.