Read Desirée Rumbaugh's response:
Cramping in the outer hip of the standing leg is actually quite common in Ardha Chandrasana (Half Moon Pose). And the cure for it is so simple that it quickly becomes a nonissue once the cause is understood. Your hunch is right-on: Students who experience this pain do need to strengthen the external rotators in the hip. And the very good news is that the best place to strengthen them is while doing this very pose—Half-Moon Pose!
When this student applies the following actions and performs them correctly there will be no pain at all. A yoga pose should never be performed if it is causing pain to a joint. You will know you are doing the pose correctly when this pain does not occur.
First, make sure the standing leg and foot are turned out, with the toes facing directly forward. When a student transfers weight onto the standing leg, the foot often turns in because the hips are tight and the inner thighs weak by comparison.
Therefore, it is important to learn how to become mindful as the weight is being transferred. The next step is to learn how to maintain muscular energy, an isometric contraction of the muscles on all four sides of the leg to the leg bone. Once this powerful energy is created and maintained, the tailbone can be scooped or lengthened, and the standing-leg buttocks muscles activated to maintain the external rotation that results. When the integration of the feet, legs, and tailbone is maintained, they all work together to provide a very freeing opening for the hips without causing any pain or cramping. Then the student can open to a bigger possibility and stretch from the inside out. When we draw in first and connect to our pelvic core, there is a greater possibility of opening up further and with more ease than we ever imagined.