Possibly or it could be a different mix of sand and cement used in these areas. Efflorescence can certainly be found in brickwork joints but this forms a powder on the surface of the joint as well as (usually) being white in colour.
it is a common missconception that this problem can be removed by brick acid but the truth is that all brick acid will do is attack the sand in the mortar what you need to attack is the lime to do this you need to use phospheric acid and not hydrochloric acid which is commoonly known as brick acid & alloy wheel cleaner trust me i was in the concreet industry for thirteen years and now own a cleaning copany that specializes in brick cleaning among other things google trucking clean manchester
