How should I treat and get rid of white algae or fungi on outside brick wall

Are you sure it's not calcium or lime build-up on the brick,not mold?

Calcium/lime deposits are a lot more common on brickwork,namely from the mortar(lime mix with sand)and common rainwater leaching the chemicals from older bricks.

A pressure washer and something like CLR,Lime away, etc.. That would bring back a certain amount of the original bricks, if not all.

Look into the chemical uses listed on the labels and find out what exactly the build-up on the bricks really is.