A lot of people report success with an animal eating valonia and then you will hear someone else with the same fish, crab, etc, report that their animal would not touch valonia. Earlier someone mentioned collection location/diet as one factor which makes sense. One thing I have also noticed is that there seems to be multiple species of bubble algae.
Someone just gave me a rabbitfish that cleared his tank of valonia (I had tried a rabbitfish previously with no success). While it cleared the valonia off of one of my clams it did not do much if anything to the valonia on the rocks. The valonia that was on that single clam seemed to be smaller, shinier, and less prone to spread around the tank.