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Could you give us any more information? Is it active at night? What is it's coloration? Does it have any eyespots? Does it have any bumps that stick up over it's eyes? Any stripes? Any particular habits? Any idea what part of the world it is from?
Here is another shot, mostly mantle. I know, the detail is crap, but he seems to know the camera already. He also got up cose and personal with an emerald crab that was in that tank... not bad for only 2 days...
This appears to be Abdopus aculeatus, a species that is flooding the market from Indonesia. They occur on shallow reef flats in Australia and Indonesia, are day active, voracious eaters, and if you have an adult (which you seem to have), a female will lay her eggs and begin brooding within weeks and males will senescence in about the same amount of time.