Firm but not bone hard lump on top of shoulder towards arm. 2.5 CM long. Under skin but on top of muscle/bone. Moves very little. Afraid of cancer.

See your doc. Sounds most likely like a lipoma, a benign fatty tumor. However you should bring it to the attention of your friendly primary care physician.
Needs to be seen. Firm but not bone-hard raises concerns about a soft-tissue tumor other than a lipoma, especially one of the sarcomas. A physician's office visit is in order now.
Lipoma. It is probably a lipoma but that can only be confirmed by physical examination.A lipoma is a growth of fat cells in a thin, fibrous capsule usually found just below the skin. Lipomas are found most often on the torso, neck, upper thighs, upper arms, and armpits, but they can occur almost anywhere and are non cancerous.Consult your physician to confirm the diagnosis of lipoma.