cyst on right shoulder

I went to the doctors today to be treated for a an infection regarding a kidney stone when I asked the doctor to take a look at peculiar growth on my back. He took a look at it and felt it and immediately told me that I have a cyst on my right shoulder. My question is, can cysts on the back or shoulder be harmful? Also, how can they tell between a simple cyst on the back and a tumor? This growth that I have on my shoulder is right on the bone and can be felt. It is extremely painful with just a simple touch and parts of the bump on my back is white. The growth looks small when you first look at it but it's not until you actually feel it does it feel quite a bit bigger than it actually looks. Any information on this would be extremely helpful.
thank you so much!

take care,

steph