Mucosal melanotic maculehttps://en.wikipedia.org/wiki/Oral_pigmentation#Melanotic_macule
Mucosal melanotic macule can be found on the oral mucosa, lip, palate, and gingiva. Mucosal melanotic macules are benign pigmented lesions that occur in the oral cavity, caused by increased pigmentation of the epithelium and lamina propria. Clinically, they present as brown, black, blue, or gray areas that are well circumscribed. Lesions are usually less than 10 mm in diameter, although they can be larger in some cases. The condition is benign.

○ Diagnosis and Treatment
Mostly benign if uniform in shape and color.

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      References Black and Brown: Non-neoplastic Pigmentation of the Oral Mucosa 30671761 
      NIH
      Addison’s Disease (Hypoadrenocorticism),Peutz–Jeghers Syndrome,Laugier–Hunziker Syndrome (Idiopathic Lenticular Mucocutaneous Pigmentation),Drug-Related Discolorations,Melanotic Macule,Melanoacanthoma,Smoker’s Melanosis,Amalgam Tattoo/Foreign Body Tattoo,Black Hairy Tongue