It’s best to get all new lumps checked by your GP. I suspect this is a lipoma, which is a harmless fatty lump often found on the shoulder. Lipomas tend to feel smooth and rubbery and, like yours, they move about under the skin rather than feeling stuck down to the muscle beneath. You do however need to be examined by your GP to confirm this. Lipomas only need to be removed if they’re causing pain or discomfort, otherwise they can be left alone. Very rare cancerous muscle tumours called sarcomas can appear as lumps although these tend to be fast growing, hard, stuck down and painful. Don’t take any chances with lumps, see your GP. I can assure you, you won’t be labelled as a time-waster.