Tiny sliced up pieces of silversides is what I fed mine, but any small piece of fish flesh will work as well. Just wiggle it in the tentacles and the nem should grab hold of it.
__________________ 180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18