I'm going to stick my neck out here & say that is run off from the render above.
To the left of the downspout at the top of the brickwork it is slightly running down as well, all the other bricks visible look to be quite clean.
Apply brick acid, let it fizz/do its thing, rinse off.
Amended - there is clear run lines down the stone between the bricks & render.
I'm not saying it isn't effervescing but it's too small an area to see if it is a problem on other areas of the building.