Combine mustard, molasses, sugar, tomato paste, vinegar, salt, garlic powder and cayenne pepper in a small saucepan. Bring to a boil, lower heat and simmer until thickened, about 5 minutes. Set aside to cool.
Brush rib sections thickly with sauce and arrange in slow cooker. Cook on low until meat is tender and easily removed from bone, 4 to 6 hours. Skim fat from sauce and spoon sauce over ribs before serving.