I have fed them blanched zucchini and blanched spinach before. Put it partially under a rock on one of their rocks and they hover over it chewing away.
Mine also LOVE frozen blood worms and frozen brine shrimp. I feed them these foods with a turkey baster, tedious, but they are slow eaters and if not the SAE and RTS would get everything.
I also feed them sinking algae wafers, just press them down hard onto a rock and they stick there.