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It looks like the common box crab, Calappa calappa. The color pattern in variable, but this spotted form is fairly common. It is found throughout the Indo-Pacific and feeds on mollusks by crushing and peeling them.
I saw one of those freediving one night in the Cook Islands. Pretty shy.
I also saw the only slipper lobster I've ever seen on that same night. And three blacktip reef sharks that wanted the fish I had speared. The first two I didn't worry about so much, but I figured that three was pushing my luck a little.
Cool addition, hope it doesn't eat anything any your tank that you really like.