In his opening statement, Musk's attorney Alex Spiro told the jurors that his client meant no harm when he hit out at Unsworth and that his words were spoken in anger.
Musk will be called to testify about the tweet widely interpreted to be a reference to a pedophile.
The boys, aged from 11 to 16, and their coach, ventured into the cave on June 23 after football practice and got caught deep inside
Thai authorities are working to with navy SEALs to run a fiber optic internet line into a flooded cave in northern Thailand where 12 young soccer players and their coach are trapped