Soma is a deity of post-Vedic Hindu mythology. The name is also sometimes used as an alternative name for Vishnu or Shiva.
One well-known use of saumya in yoga is that Swami Sivananda addresses his students as “He saumya” in some of his books, which can be taken to mean “My dear.” To be described as saumya is to suggest that the person shares the spirituality and depth of kindness of Soma. This encapsulates the idea that saumya is a deeply auspicious and graceful concept, full of inner beauty and light. It is also considered quite a mild quality - one that is gentle rather than aggressive and, thus, has a softer emotional effect and meaning.