i had a foxface that started picking on mantles after several months in the tank (about 6 mo's).
Foxface is gone.
What probably happened with my foxface is that as it grew larger it felt hungry and just started picking on whatever it could find in the tank, as algae was scarce at the time, what else to go for but a nice teardrop (actually they probably taste rather nasty).
Just my experience.
By the way, I had a kole go after the clams after almost 2 years of best behavior. Probably for the same reason.
Omar