Here's an article that you might find helpful.
That is true. Thought it was outside.
check that pin https://www.pinterest.com/pin/519462138265279556/
Usually muriatic acid is used to clean brick but I am not sure about the white color. You could try an inconspicuous area first. Be sure to use protective clothing and eye wear as well as putting something on the floor to protect that surface.
Get some Paint n Peel, paint it on with a small roller or large brush, then peel off all the soot and dirt....https://www.youtube.com/watch?v=1jFlqynHYcU