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If it is fairly new mortar use a 50/50 solution of muratic acid and water. Wear gloves and eye protection. I use a brush and have the hose handy to rinse. Just occurred to me inside or out? The hose for rinsing may be an issue...
F'ing around with muriatic is not something I recommend to anyone, it is a 30 percent solution of hydrochloric acid and is a serious industrial chemical. If you insist on using it, hose down the entire area until fully saturated, dilute to a reasonable mix (10 to 15 to one) by pouring the acid into the water and wear appropriate safety equipment.
There are available proprietary cleaners that are a little safer, Sure Klean 600 detergent is one, although it is still a dangerous product. If you are not able to keep from making a mess while laying your masonry, then you can use industrial strength vinegar to remove minor amounts of mortar.