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Actually, it looks like a snail (stomatella varia) and they are awesome film algae grazers. If you find any more, keep them. They are worth it thier weight in salt.
I agree with Fluff. Stomatella, they can quickly populate a tank. I have tons of them in my system. You can see them spawn too. Pretty interesting little critters. Back to the isopods the one you took a pic of are ok to have. They are just "reef" bugs and free food for your fish. As Scott said the ones with large black eyes are the bad guys.