Whisk 1/2 cup water, the coconut milk, curry paste, fish sauce, ginger and lime zest in a 6- to 8-quart slow cooker. Smash the lemongrass stalk with the flat side of a knife or a meat mallet and tie in a knot. Add the lemongrass, peanuts and potatoes to the slow cooker. Top with the chicken thighs and gently press to submerge in the liquid. Cover and cook on low, 7 hours.
Uncover the slow cooker and skim off some of the fat. Stir in the lime juice; season with salt. Break up the chicken into smaller chunks. Let stand 5 minutes, until the sauce thickens slightly; discard the lemongrass.
Serve the curry with rice. Top with peanuts and cilantro and serve with lime wedges.