If your mystery snail is an Apple Snail, I think it is, then I don't give mine any special food. I have rarely seen it eat, although it must do! They eat algae in the tank and also any fish food which has fallen to the substrate. If you type Golden Apple Snail or Apple Snail into a search engine you can find lots of information. I would do a link for you but unfortunately I am fairly useless at computers! I have tried putting cucumber in the tank for the snail and my Common Pleco but they ignored it. I also tried lettuce with the same result.
Thanks, I already put sinking pellets and Algae Wafers for my Common Pleco. Perhaps the snail has been eating that? It is certainly thriving! I have also tried cucumber and lettuce and none of the tank's occupants are interested at all. I do have real plants.