The Mobile feat (PHB, p. 168) says:
When you use the Dash action, difficult terrain doesn't cost you extra movement on that turn
The monk's Step of the Wind feature says:
You can spend 1 ki point to take the Disengage or Dash action as a bonus action on your turn...
Due to the wording here, would that mean that if a Monk with the Mobile feat uses Step of the Wind to Dash, are they not affected by difficult terrain?