They don't seem to..
I have a bunch of hydroid-rocks in this tank and the intention was to see if they'll clean the rocks.
They do sort of clean the tank, I don't really have any snails in this tank other than maybe a few Stomatella. But they don't seem to go after the hydroids.
To be honest I'm not sure what they're eating. Occasionally I'll try to feed them some mysis by placing some under them but they don't seem to eat the ones I give them. Maybe I need to offer bigger pieces (shrimp or prawn?). I've had them at least 6-8 months now if not even longer and I've never really seen what they eat. I hope I'm not starving them..
I also have a cowry in this tank that I have suspected of munching on soft corals, zoos, etc. in the past. Since moving the cowry into this tank from the other, suddenly clove polyps, a few remaining single zoo polyps, that were just "hanging on" before, have started popping out of nowhere and growing. So this tank is I guess the home for all my "non reef tank suitable" creatures.. it will be the coral-less tank.
I put an urchin in there this morning, and it occured to me afterwards that I don't know if that will be a problem or not (with the CC stars). Hopefully not. If there is, hopefully I can move the urchin over to another tank in time.