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:
Beautiful painting by Zeng Hao! Green Tara - originally a Hindu goddess and later adopted by some forms of Buddhism. She is known as the mother of liberation, a protectress, an obstacle remover, and an earth mother.