Mokka — sometimes Mocha, adding to the confusion — is a variety of Yemeni origin producing exceptionally small, round beans.
The beans
A fraction the size of normal arabica, and nearly spherical. They look more like peppercorns than coffee.
The roasting problem
Very small beans roast much faster than normal ones, and mixing them with anything else guarantees uneven results. They need their own profile and their own batch.
The cup
Complex and concentrated: chocolate, winey fruit, spice. It carries recognizable Yemeni character.
The yield problem
Extremely low. It is grown for interest rather than economics, principally in Hawaii, Yemen and a handful of Latin American farms.
Not the drink
Nothing to do with a caffè mocha, and only distantly related to Yemeni Mokha the port. The naming in this corner of coffee is a mess.
Brewing it
Filter or immersion at medium roast. The small beans drop into a burr grinder differently from normal coffee and can make it run faster, so check your grind and start a step finer than usual. Hawaiian lots from Maui are the most likely to reach a US specialty roaster, usually in very small bags.