Friend, foe, or neutral?

[Puzzles386]

I'm assuming this is a flatworm of some sort, but could someone tell me whether it's likely to cause a problem? Looks light tan, oval, and about 3x2mm. I have not seen any on fleshy tissue of corals, but they have been in small rocks with gonis and on the glass at night.

 

[ISpeakForTheSeas]

That's either a limpet, a chiton, or a Seroloidean Isopod (A.K.A. trisobite or sand skater) - either way, their likely harmless/beneficial.

Limpets have a solid shell and a snail foot; chitons have a segmented shell (like a rolypoly) and a snail foot; and Seroloidean isopods have a segmented shell and little legs.

 

[Puzzles386]

That's either a limpet, a chiton, or a Seroloidean Isopod (A.K.A. trisobite or sand skater) - either way, their likely harmless/beneficial.

Limpets have a solid shell and a snail foot; chitons have a segmented shell (like a rolypoly) and a snail foot; and Seroloidean isopods have a segmented shell and little legs.

Thank you. They don't appear to have a solid shell, more of a segmented body. I was worried that they might be the cause of several goniopora receding, but it sounds like they're not the culprit.