i feed mine collard greens
collards are very high in calcium, which is very important for healthy shells
for a list of greens high in calcium do a google search for iguana food, as iguana need alot of calcium too and much more research has been done on their diet than for snails... hate to say it, most snail keepers think the snails eat poo
i see you are in florida, you should have a great growing season to grow your own collards, i throw a few seeds in the corner of my flowerbed every year for mine, but they dont get all that big cause of the short growing season