Cucumber, zucchini, lettuce (NOT iceberg), spinach or carrot. Never leave vegetables in the tank any longer than 8-12 hours as it will start to rot and ruin your water quality. Apple snails don't like to eat algae they will eat it, however, when they are hungry. If you're looking for an algae eater in particular nerite snails actually like and eat up algae really quick.
My lights are only on about 8-1/2 hrs a day. As for feeding my snails, I feed algae wafers & a few times a week I drop zucchini (removing the seeds) & kale. The kale gives them calcium which they need for their shells. The 2 snails in my 46g tank don't usually touch the veggies the 1st day but it's completely gone within 48 hrs. In my 2 10g tanks, it's mostly the same & I've had no water issues leaving the veggies for that long.