Stomatella snail good guy
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does it have a flat shell
It's definitely a flatworm. I'd try to get it out and put it in a sump.
Yeah just watch him. He's probably harmless if he just came with the rock and not hitched on a coral.
But just keep an eye out. Also watch to see if you see snails disappearing. I know there are flatworm species that prey on them.