Mucocelehttps://en.wikipedia.org/wiki/Oral_mucocele
Mucocele is a condition caused by mucus extravasation phenomenon or mucous retention. The mucocele has a bluish translucent color, and is more commonly found in children and young adults.

The most common location to find a mucocele is the inner surface of the lower lip. Some mucoceles spontaneously resolve on their own after a short time. Others are chronic and require surgical removal.

  • Mucocele treatment
References Overview of common oral lesions 36606178 
NIH
The pathologies covered include recurrent aphthous stomatitis, herpes simplex virus, oral squamous cell carcinoma, geographic tongue, oral candidosis, oral lichen planus, pre-malignant disorders, pyogenic granuloma, mucocele and squamous cell papilloma, oral melanoma, hairy tongue and amalgam tattoo.
 Oral Mucosal Lesions in Childhood 36354659 
NIH
Mucocele develops as a consequence of mechanical trauma to a minor salivary gland, which is followed by saliva retention and accumulation inside the blocked and dilated excretory ducts of the gland. Lesions are usually painless, with smooth surfaces, bluish or transparent. Most are not larger than 1 cm in diameter. They are treated by surgical removal; at that time, the surgeon often decides to perform the ablation of the neighboring minor salivary glands in order to prevent relapses.