would have helped to do a search first...didn't realize there was an entire post on snail ID's!
COLLONISTA sp. (Mini-Turbo Snails)
Family: Turbinidae
Range: Indo-Pacific
Size: Up to 1/4"
Diet: Microalgae, diatoms
Reef Safe: Yes
Temperature: : 72 - 78ºF
Specific Gravity: 1.023 - 1.025
pH: 8.1 - 8.4
Care Level: Easy
Notes
These snails are almost exclusively introduced into the tank as a hitchhiker on live rock. However, once they become established it is not uncommon for them to breed and multiply; completely eliminating the need for other snails.
This highly beneficial snail can be easily identified by its very small size and a distinctive pit in its operculum.
Care must be taken if you have a proliferation of these in your tank. Due to their small size they can become stuck in pump intakes, and can cause auto-topoff systems to get stuck in the "on" position