It's a planarian (flatworm), maybe Bipalium kewensis.

Joe

I'm familiar with planaria but didn't think they would exist in my garden! I'm wondering if this is a good thing. As I recall they are carniverous. I'll go google.

Tx Joe.

They eat earthworms, so in my opinion aren't desireable.

That critter is a common slug without the shell. Take a plastic baggie with a bit of table salt toss the critter in and he'll bubble and be gone forever as long as you get the pesky snails and other slugs out of your garden in the salt. They chew big holes in the succulents.