Mine was a royal PITA to feed.
After several long weeks of attempting silversides, mysis & brine shrimp I gave in & fed him a variety of different live fresh water shrimp & ghost shrimp.
I fed the shrimp on selcon & garlic soaked foods for added nutrition.
As & when I could get it, I also fed live brine shrimp, which were also gut loaded with selcon beforehand. The frogfish eventually got used to associating brine shrimp as a tasty treat & would occasionally eat that too when I fed the other fish in the tank.
He only ever ate silversides about 2 times in 2 years.
Feeding this thing has cost me a small fortune!
.