Simples any mortar stains can be removed with brick and mortar cleaning acid,sold over the shelf at most builders merchants. Be sure to wet walls down with hose first, then apply acid with paint brush to stains moving brush to and throw, you'll see stain bubbling as acid works,dont leave on for more than 20 mins before washing off with hose.
Try a small area first out of sight to see that it doesn't affect any finish you may have, but shouldn't do.wear safety goggles and gloves if you get on skin it will tingle not burn but wash off immediately anyway.

Hose down the wall first be sure to only paint the brick with acid as can cause discoloration if bed joints and perps are caught be sure to rinse off well after 15 to 20 mins read the instructions as you maybe advised to dilute wear gloves and goggles.

Brick acid you can get at any hardware store, wet down scrub in and wash off be sure to wear gloves and goggles as it’s highly corrosive

你好,你可以买到现成的迫击炮去污剂or brick acid scrap the mortar off with a scrapper then dilute the acid to the manufacture guidelines make sure you have to correct Ppe and a decent bucket scrubbing brush wet the wall and apply the acid with the brush and wash down well please note this could cause the salts in the brick to come out on the surface

Hi you can buy brick cleaner or brick acid from all good builders merchants making sure you wet the area use a scrubbing brush and all the correct ppe work the acid into the effected area leave to soak in for 20 minutes then wash area thoroughly as not to leave streaks