My mystery snails- we have 4 in one tank and 1 in every other tank- are given one whole Omega One sinking Algae wafer a day. They're in a fully established tank that is fully planted and also houses one betta, one pictus cat, and one shrimp. The water is heated, filtered, and fully cycled and established. They do perfectly well, we've had two die of age so far, and that's all. We actually now have 5 egg clusters (one is hatching right now). The shell color/condition is a great indicator of the snail's health, good coloring usually means you have a healthy, happy snail. If the colors dull or start to look "gross" then it's feeling sickly. They also go through a stage every now and then in which they simply go dormant for a day or two. This seems to be normal and necessary for their health. We almost thought one was dead the first time one did this, but since the shrimp wasn't trying to eat it yet we let it be and it eventually woke up and was her perfectly active normal self.
Mine do not like veggies...they like shrimp pellets and algae wafers, that's all I can get them to eat. The wafers provide a lot of calcium, and that is important for healthy shells. Just not too much, since a larger amount of calcium in any invertebrate is unhealthy.