A very small shrimp. They live on wire coral (hence the very original name). To give you some idea of scale, the wire coral's diameter is about half your little finger's.

These guys are hard to spot, partly because of their size but also because their colour matches that of the wire coral. Which doesn't show in this shot. In one of those "physics is cool" effects, the strobe light affects the two things differently. Without the strobe, they both look the same green. The strobe brings out the hidden red in the wire coral, while the shrimp is just green. Since predators don't carry big cameras and strobes, the red in the wire coral does not reflect any light (the red light from the sun is absorbed by the water after about 3m/10' of depth) and thus does not usually show.