Apple snails will eat just about anything, but they do like protein-rich foods like those algae wafers (which contain a good amount of fish meal for protein) better than straight veggies. You can try breaking the wafers into several pieces so the snail gets a piece and so do the catfish.
Green bell pepper is probably good for them. For any hard veggies I want to soften for my apple snails, I wrap them in a damp paper towel and microwave for 30-60 seconds until they are a bit softer. I'm especially fond of carrots for apple snails because a slightly cooked carrot will sink and lasts for almost a week.
Most veggies you'd put on your salad are good for apple snails. Soft lettuce, carrots, cucumber, etc. are all good. I bought some feeding clips that I think are designed to feed marine fish seaweed, but I just clamp it to the lettuce or cucumber and drop it in the tank to make it sink. Best investment for my snails ever.