No - this is quite serious, in spite of it's (regrettable) racial connotations. This is quite common terminology in the South East of England and London. "Nigga" refers to the blackness of the cloves (smoked in similar style to the Indonisian Kretec), whereas "cracka", or "cracker" (a racial term used against Caucasians) refers to a J devoid of "blackness" (ie, cloves). Niggas tend to smoke slower and have more flavour, whereas crackas smoke faster and are more intense (in my inexpert opinion....).