Sally Light Foot Crab
This is a favorite of the reef aquarist because it is a reliable and non-aggressive cleaner. Here it has its rear against the glass, facing away from us.
Seen from the front, you can just barely make out a flickering "eyebrow" on the rightmost of the two eyes.
Trying to climb the glass, we get to see its belly clearly. A banded coral shrimp is in the background.
Here the Sally tries to climb the glass again, and we get to see it and it's reflection at the same time.
No, this one did not die. Since crabs molt periodically, if you catch the molt quickly enough, you can assemble a "crab kit" fairly quickly.
A close-up of the face, mandibles, and eyes.