Live foods is a good place to start. I would add some cyclopeeze to the brine shrimp and let it sit with it for a good 10-15 min. It will gut load them and make them pretty nutricious. You could also try some fresh water glass shrimps and gut load them the same way. That is what I feed my 2 angler fish and they can't get enough of it.
In my limited experience with 2 foxface, I have found them to be voracious eaters. I added on to my previous 360g tank to eat bryopsis and it did not touch it. I added a different one a few weeks later and got the same results. However, the fish tripled in size in a year because they would eat so much of the frozen mush I used to prepare. Their poop production did not triple, but rather was multiplied by 10!!!!!!!!!!!!!
Give it some time. That is certainly a very nice looking fish. Is it even picking at the rocks?