I know everyone wants to power wash brick, but it's really not a great idea and can cause more damage (mostly cosmetic) than you've already got. Especially if you don't know what you're doing. I used to build homes and we had so many issues with people blowing the face off the brick with power washers, and once you do that you're kinda SOL.
There is a company called Prosoco that handles masonry specific cleaners. They could advise you on some materials and methods to get that clean. Might even send you a sample, which would be more than sufficient for cleaning that area.
If you do power wash, start by standing way back and use a wide tip.
