Pleomorphic Adenoma: This is the most common type of benign salivary gland tumor, and its presentation matches the description given. It is known for its slow growth, mobility with muscle contraction, and variation in size with jaw movement due to its possible connection to the parotid gland or other salivary glands.

Mucoepidermoid Carcinoma: Although less common than pleomorphic adenoma, mucoepidermoid carcinoma can present as a painless mass and is a consideration in the differential diagnosis of salivary gland tumors. However, its behavior can range from benign to malignant.
Warthin Tumor: This is another type of benign salivary gland tumor, more commonly found in males but can occur in females. It might present similarly, though it's less likely given the patient's gender and the specific characteristics mentioned.
Adenoid Cystic Carcinoma: While this is a malignant tumor, it can present as a slow-growing, painless mass. However, it is less common and tends to have a worse prognosis compared to the other options listed.

Parotid Gland Malignancy: Although the presentation suggests a benign process, any mass in the parotid gland area warrants thorough investigation to rule out malignancy, given the potential for serious consequences if missed.
Metastatic Disease: Rarely, a metastatic lesion from another primary site could present as a subcutaneous mass in this area, emphasizing the need for a comprehensive diagnostic workup.

Benign Lymphepithelial Lesions: These are rare and can occur in the context of autoimmune diseases like Sjögren's syndrome, but the patient's lack of medical or autoimmune history makes this less likely.
Salivary Gland Hemangiomas: These are rare, benign vascular tumors that could present as a mass but are more commonly diagnosed in childhood.
Neurogenic Tumors: Such as schwannomas or neurofibromas, which are rare in this location and might not typically move with muscle contraction, but could be considered in a broad differential diagnosis.