Whisk the lime juice, crema and 1/2 teaspoon salt together in a large bowl. Add the cumin, scallions and garlic and let stand 5 minutes. Add the cilantro, cabbage and jicama and toss to coat. Let stand for 15 minutes, tossing occasionally. Serve immediately or refrigerate up to 1 day (the cabbage will release its color and turn the slaw pink over time).