Chris, What is the time period on your building? I agree, most likely what you are seeing is some type of iron oxide pigment in the mortar. If your structure is pre-1900 you are more than likely dealing with soft lime mortar. If you would like you may send me a small sample of the mortar and I will be able to tell what you have. Just send me an e-mail and I will give you my address. Steve Elkins Savannah, GA.