Here, the phrase so little faith means the same as no faith.
faith the size of a mustard seed: This suggests that even a tiny amount of faith can help the disciples achieve great things (Hebrews 21:21; compare 1 Corinthians 13:2). It also suggests that if they let their faith increase (see Matthew 13:31–32), they could act with strong belief.
"Moving mountains" is a saying that means overcoming obstacles and achieving God's goals (Isaiah 40:4; 49:11; 54:10; 1 Corinthians 13:2).