Mortar in Mosaic Grout Lines
Make a weak acid solution using Muriatic Acid (pool acid) and water in a 5 gal. plastic bucket, 1 cup:gallon. Using gauntlet type chem gloves and eye protection, scrub with acid. Use a grout sponge to rinse and wipe clean, using clean water and wringing frequently. Repeat until all excess mortar and haze is gone.

Note: above method is effective for removing dried smears and haze, solid clumps of dried mortar must be mechanically removed (scraped).