Mucous cysthttps://en.wikipedia.org/wiki/Mucocele
Mucous cyst are benign ganglion cysts of the digits, typically located at the distal interphalangeal joints.

  • It typically appears as a soft lesion without symptoms on the lips.
  • It may also appear in the distal joint of toes or fingers.
References Digital Mucous Cyst 32644518 
NIH
Digital mucous cysts are a type of ganglion commonly found on the hand. A ganglion is a soft tissue tumor that is found next to a joint or tendon. A digital mucous cyst is a ganglion that arises from the dorsum of the distal interphalangeal joint (DIP joint). Digital mucous cysts commonly have an association with underlying DIP joint osteoarthritis.