Never had mocha mousse, but perk up your favorite chocolate mousse recipe with some coffee powder or Kahlua and you should be good to go.
The Herve This recipe with espresso or coffee instead of water as the base, for example.
https://www.foodnetwork.com/recipes/death-by-chocolate-mocha-mousse-recipe-1908600
https://www.foodnetwork.com/recipes/sandra-lee/mocha-mousse-2118766
