What is this?

[Crystal_]

Very new to marine aquariums, and have been preparing my current tank for clownfish and corals, hopefully in a few weeks. I added live rock just over a week ago, and have noticed this little guy today. Wondering if I should be worried or if he's a helpful critter? Thanks in advance

 

[Rtaylor]

Stomatella snail. Great hitchhiker!

 

[sfin52]

Agree. A fantastic hitchhiker.

 

[WheatToast]

Might also be a chiton. I can almost make out the interlocking plates on top of the animal in the first picture and the mantle extending over the side of its foot in the second picture, which are indicative of a chiton. I also can’t make out the tentacles and beady eyes characteristic of Stomatella, but this might be because the animal is retracting at the water line. Like Stomatella, chitons are beneficial grazers that will multiply over time.