No need to panic, but you should have it checked out by a doctor. Most moles do not cause any problems. If it was at all raised, or dark in colour or has changed in shape it needs to be checked. The fact that you caught it with your nail probably means it was raised. Was it the mole that was itching at the time or your shoulder in general? I had a raised, black mole taken off my back as a child. Cancer does attack certain moles and if yours has been damaged, that means it is weakened tissue, so might be more prone to develop into a cancerous mole. So, rather than worry, have it removed. It is a quick and simple procedure that can be done in the surgery and doesn't hurt. Most doctors prefer to see them removed, but some still think it is not always necessary. In my case, it was taken off a bit too late and I did develop cancer years later. This is not what you want to go through, so better so be safe than sorry. Best wishes.