I would make sure that they aren't Giant African Snail babies. They eat just about any kind of plant and sometimes they eat stucco to get extra calcium in their diet.

Diachenous earth (can't spell it!) is made of crushed sea shells. It will kill them if your spread it around on the ground where you see them. It is pricey IMO. Don't use it on a windy day and you should use a mask when you spread it. It can hurt your lungs.

Have them en mass. And yes, they eat plants, specially young greens. DE should work, they won't cross over it, but it'll be gone after a rainfall. They are a BOTHER when you walk to your car and crush them underfoot.....got me used to always check the ground where I step.

I use snail bait in areas where snails and slugs are a problem. To keep the rain off the bait, I pour a small amount in a plastic cup, turn the cup on its side and let the snails have a party. I empty it when it's full of the little devils.

I've come to rely on the pelleted bait - I often sprinkle it around new plantings of seeds to keep the slugs & snails from destroying the seedlings before they can take hold. I had a bad infestation of Cuban brown snails a couple of years ago & now they're almost completely gone from my yard.

I buy it @ Home Depot - most garden supply stores should have it - it's iron phosphate & breaks down into fertilizer...

Thinking those in the photo might be tree snails? This article says they are not a pest...

Here is a link that might be useful: Tree snails