It is possible that this lump on your back maybe a calcium deposit. However, given the limited information in your question, it is entirely possible that this maybe something else. Most commonly, a lump on the back that is soft, under the skin, and mobile maybe a fatty benign tumor, known as a lipoma. Generally, these can be diagnosed with exam and can be removed if causing discomfort or for cosmesis. In addition, it maybe a sebaceous cyst depending on what it looks like and your clinical history. However, more importantly it is important that you see a doctor
to rule out a skin cancer that classically presents with ABCDE's or asymmetry, changes in borders of the mole, change in color , has a diameter more than 6 mm or is elevated. Important to know is whether the spot on your back is changing with these characteristics over a period of time. It is important for you to follow up with your primary care physician
to have this lump examined so that your can be diagnosed accurately and be treated appropriately. Your doctor will be able to ask questions about this lump, examine it carefully, and determine if biopsies are needed in case the lump is suspicious looking.