What is the Buddhist term for each moment being subtlety unique? Read it a long time back, I remember it had something to do with someone meditating in a field of grass, and watching the wind go through the individual blades of grass and how they swayed. 

It was a specific term and dealt with emptiness and the uniqueness of each passing moment.
Maybe the term is "Tathata" that means "suchness" or "thusness". It is a Mahayana term. It means "Things as they are" or "reality". It's ultimate reality as opposed to conceptual reality. 

It comes from the word the Buddha used to describe himself," Tathagata" that means "one thus gone".


  In Chan stories, tathātā is often best revealed in the seemingly mundane or meaningless, such as noticing the way the wind blows through a field of grass, or watching someone's face light up as they smile.
  https://en.wikipedia.org/wiki/Tath%C4%81t%C4%81