Mystery snails can be very voracious eaters and can quickly decimate aquarium plants. Here are a few steps you can follow to help keep your mystery snails from nibbling on your Amazon Sword plants:
Add Floating Plants: Floating plants, like the Amazon Frogbit, adds variety to the diet of your mystery snails and deters them from grazing on your Amazon Sword.
Supplement with Veggies: Try supplementing your snails' diets with small pieces of vegetables, like lettuce and blanched spinach. You could also try veggie sticks or frozen blanched veggie cubes.
Plant Burrows: Planting burrows such as Extra-High Tech substrate or AquaScapes Aquarium substrate give your mystery snails a hiding spot to retreat to when they're full or have enough to eat.
Increase Snails: If you have a number of mystery snails they may be competing with each other over a limited food source, which is the Amazon Sword. Consider introducing additional snails to reduce competition and reduce the likelihood of them being attracted to the Amazon Sword.
Positive Reinforcement: If you notice your snails nibbling on the Amazon Sword, intervene before they’ve done too much damage. Take your snail out of the tank and place it in a seperate container with plenty of blanched veggies or veggie cubes; this positive reinforcement can help train your snails to focus on the available food.
I hope this helps to reduce the likelihood of your mystery snails nibbling on the Amazon Sword plants in your tank. For more information, please refer to this article: https://www.fishkeepingworld.com/how-to-stop-pests-from-eating-your-aquarium-plants/