What's the best method for removing mortar stains from faux rockwork? The mortar is about a week old and there is a residue that covers the rock and prevents it from matching the surrounding rockwork surfaces.

First try something not too strong like spaying with 50/50 vinegar and water and allow to soak. If its outside power wash from about 12" away. If it doesn't work try staright vinegar before we move to diluted muratic acid.

Diluted muratic acid, it will eat the rock right off your mortar lol
Wear some gloves and safety googles.

I'm a little hesitant to use muriatic acid... it IS faux rock.

You May Try A Course Scrubbing Brush And Use Something A little Less Toxic

Thanks for all of your suggestions. I ended up using an architectural concrete cleaner/brightener. After I got to the job and read the label further... I realized that I had spent $20 on a dilute muriatic acid. It worked pretty well, though. For some reason it bleached the grout somewhat but I ended up with a satisfied customer. Now... if I could only get paid for the job. It was one of those darned "escrow" jobs.