Any algae wafers that it will eat. I use hikari algae wafers. I tie them to a small piece of sps skeleton and put it in front of the conch. Both conch and other snails enjoy it. Mine didn't eat nori or flakes, even tho other snails devpured it. I also recommend getting small active individuals, because i had one for a year, but it was flegmatic from the start, stopped eating much for a few months and then died. I think it was dinos and cyano that got it. The new one is much more active and i don't have as much dinos and cyano. I hope diatoms will bloom instead, at least they are a food source for animals.
PS: if it can't find enough food in the sand, the conch can start eating algae from the rocks and from the glass.