Sebaceous cyst: Given the description of a mobile, ovoid hypoechoic mass with a possible tract extending to the skin surface, a sebaceous cyst is the most likely diagnosis. These cysts are common, benign, and often have a characteristic appearance on ultrasound.

Lipoma: A benign tumor composed of fat tissue, lipomas are common and can present as mobile, soft masses under the skin. They might not typically have a tract to the skin surface but could be considered given the mobile nature of the lump.
Epidermoid cyst: Similar to sebaceous cysts, epidermoid cysts are benign and can have a tract to the skin surface. They are less likely than sebaceous cysts to be the correct diagnosis based solely on the information provided but are still a plausible option.
Dermoid cyst: Although less common, dermoid cysts could present similarly, especially if they have a tract to the skin surface. However, they often contain elements from all three germ layers, which might not be evident on ultrasound.

Squamous cell carcinoma or other skin cancers: Although less likely given the benign appearance on ultrasound, it's crucial not to miss malignant diagnoses. Any lump with a tract to the skin surface could potentially be a sign of a more serious condition, especially in older adults.
Infected cyst or abscess: An infected sebaceous or epidermoid cyst could present with similar ultrasound findings and would require prompt treatment to avoid further complications.

Pilomatrixoma: A rare, benign tumor of the skin, pilomatrixoma could present as a lump but is less likely to have a characteristic tract to the skin surface.
Metastatic disease to the skin: Rarely, metastatic cancer can present as a lump in the skin. Given the lack of other symptoms or history provided, this would be a less likely diagnosis but should be considered in the differential, especially if the patient has a known history of cancer.