*thinking they might be getting in through the bathtub drain. I hang the board from tub faucet while watering it and let it drip dry. Will run a tubful of hot water through drain, but I'm hoping to rid all of the little snails that might be on the plant and in the moss still.

More likely that eggs have hitched a ride on anything that you might have added to the bed in the last few months, or when you water with a container that sometimes sits outside. Doesn‘t look like your Bonide will work for snails; you may have to get something specifically designed for snails and slugs.

I did add some mosquito bits to help with fungus gnats, maybe they had some eggs. Will try some snail/slug bait.