what is eating the leaves on my calla lily?
It's not slugs.
There are many things that could be eating your calla lily leaves. If you've eliminated slugs, that still leaves a host of caterpillars, grubs, beetles, and grasshoppers. You can do some detective work at night with a flashlight, or dig around in the soil to see if anything is curled up there. Even if you can't identify the culprit(s), you might try spraying the plants with neem oil. https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm