It's the Tibetan syllable TAM, the seed syllable of the female buddha Tara (sanskrit) or Dölma (tibetan), which translates as Liberatress. As it is white here, it stands for the White Tara, which especially gives the blessings for a long life:
Donald Carol Dakinis A dakini (Sanskrit: “sky dancer”) is a Tantric priestess of ancient India who “carried the souls of the dead to the sky”. This Buddhist figure is particularly upheld in Tibetan Buddhism. The dakini is a female being of generally.