The successful broodstock (breeding pair/group) diets of various conchs that I've heard of all use some kind of pellet/gelatin feed that was supplemented by/mixed with algae (specifically algae like Ulva, Halymenia, and Spirulina).
So, I'd get some good pellets (I'm not sure what is best for conchs - I know Mazuri Koi Chow was used successfully with four conch species, so that would be a safe option, but personally I'd give Otohime or TDO pellets a try) and some good macroalgae (preferably red and/or green - again, I'm not sure which algae would be best, but I know Ulva and Halymenia have been used successfully, and I know some conch species eat Laurencia and Batophora in the wild; Ulva would probably be one of the easiest to get a hold of and grow out so you don't have keep buying more) and some Spirulina powder and blend these into one feed you can offer the conch.
Basically take some pellets, macro, and Spirulina and turn them into one pellet/gelatin (or possibly even frozen, though I've never seen frozen used) feed to offer the conch (you may need to get creative in how you offer the food to the conch so your fish don't eat the food before your conch does, but you might be able to bury the food in the sand for it or offer the food to the conch inside an isolation box or something).
