Trying to target a specific algae with a specific fish can be dicey at best. You simply cannot rely upon them to eat what >you< want them to eat. People can get lucky and have a fish eat something, which gives rise to the suggestion that all fish of that species will eat that algae. It isn't necessarily a problem if the tank in question has enough space, alternate food sources, etc. In cases like yours, though, even if the Foxface you get eats the valonia, the fish may already be too large for the tank or may outgrow it in short order. Then you've got a stressed/dead fish, or a venomous fish to try to remove from your tank.
FWIW, I have had a few Foxface and Rabbitfish over the years, and I've never seen one do more than pick at valonia once or twice. I've never seen anyone recommend a fish that will reliably eat the scourge.