Generally little snails that look a lot like conchs, under half an inch at maturity, are good guys; but there is now one loose in the hobby that is NOT good. It looks exactly like the good one except when you pick it up and look at the underside the snail body bunches up in the middle of the slit.
They are nocturnal in their predation. Generally if you see little snails clustered in the vicinity of a coral that has been having trouble, before lights-out, I advise getting them out. I know they are predatory on montipora, which attracts them like bait, and is my method of getting rid of them: a cheap frag mounted atop the rock will draw them there. And then you just have to be watchful. I've also seen them on acan, and annoying the daylights out of a clam. I have a lousy camera, so no pix available.
They are nocturnal in their predation. Generally if you see little snails clustered in the vicinity of a coral that has been having trouble, before lights-out, I advise getting them out. I know they are predatory on montipora, which attracts them like bait, and is my method of getting rid of them: a cheap frag mounted atop the rock will draw them there. And then you just have to be watchful. I've also seen them on acan, and annoying the daylights out of a clam. I have a lousy camera, so no pix available.