Epidermoid cyst: This is a common benign lesion that can present as a small, hard lump in the midline of the scrotum, not attached to the testicle. It is usually filled with keratinous material and can be diagnosed with ultrasound.

Lipoma: A benign tumor composed of fat tissue, which can occur in the scrotum and present as a small, hard lump, especially if it becomes fibrotic.
Dermoid cyst: Similar to an epidermoid cyst but contains more complex tissue elements like hair, skin, and sometimes even teeth. It can also present as a hard lump in the scrotum.
Cyst of the epididymis: Though less common in the midline, these cysts can occur and may feel hard if they are filled with a thick, gelatinous material.

Testicular cancer metastasis to the scrotal wall: Although rare, testicular cancer can metastasize to the scrotal wall, presenting as a hard lump. Given the potential lethality of testicular cancer if not caught early, this diagnosis must be considered.
Scrotal wall lymphoma: Lymphoma can manifest in unusual locations, including the scrotal wall, and would be critical to diagnose early due to its potential for aggressive behavior.

Scrotal calcinosis: A condition characterized by the deposition of calcium in the scrotal skin, which can present as hard lumps.
Foreign body reaction: In cases where a foreign object has been introduced into the scrotum (either intentionally or unintentionally), a hard lump could form as a result of the body's reaction to the foreign material.
Sperm granuloma: Typically associated with vasectomy, this is an inflammatory reaction to sperm that can form a hard lump, though it's more commonly associated with the vas deferens rather than the midline scrotum.