First thing to note, these are bad guys. If you see them in your tank, remove them.
Sundial snails are small, predatory snails that enjoy feasting on Zoanthids. They will actively hunt them.
If there are no Zoanthids in your aquarium, they will move on to other coroals, mainly soft corals and LPS corals.
They can enter your aquarium as hitchhikers on live rock, and are typically a white colour with a black spiral pattern, some specimens will have a slightly orange tint to them too.
How to control the numbers
As with many of the snail species, it is best to physically remove the sundial snail as soon as you see it.
As with most things, taking a pro-active approach before putting anything into your tank is best. Where possible, dip and quarantine anything that is going into your aquarium. Quarantining your liverock and coral will give you time to spot any potential threat and remove it before it goes into your display tank.
Sundial Snail 
Sundial Snail
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