slug

Hey There: Does anyone know anything about these? I heard sea slugs aren't the best bc of toxins. Is this true? Or can I safely have one of these in my reef tank? Thanks

Recently, I noticed one of these crawling around on my glass at 2am, (my lights were obviously off) any idea what it is? I noticed a second one the other day, I never see them on my coral. They seem to eat algae so I'll let them stay. :beerchug: