Place cayenne into a food processor, add garlic, shallots, cilantro, lime zest, lemon grass, salt and shrimp paste. Process into a paste. In a saucepan, simmer coconut milk over low heat until it reduces to half. Transfer paste from food processor into the coconut milk. Simmer for 5 minutes. Season with fish sauce and sugar. Stir until sugar is dissolved. Add crab and stir until crab is done. Transfer to a serving bowl and sprinkle with cilantro.