I think our snails are something different. I looked at one very closely and it doesn't really look like a sundial. Makes me think of nerite-like instead. One thing for sure, my snails are the same as badx's snails. Although I didn't get them from him, nor he from me.
I have just 2 zoanthid colonies in my tank. One is actually spread around the tank (rearranged the landscape last year). My green zoas are very happy, especially now that they're out from under the intense light zone. My yellow polyps have been shrinking and looking more tepid as time goes by. I feed them, but I think they don't like their location (also rather intense light). I'll move them when I do my next rearrange.
Anyway, I don't see my little snails on either zoa, and my green zoas are thriving. Evidence that our little snails are not that mean sundial. Still don't know what they are, or how I got em.