Tremendous technique. Looks awesome. I like the idea but I think you could push it even more:
The shape is actually not that different from regular turtle because the coral follows his shell and its all the same size. Why not have interesting coral shape with lots of holes and things in it sticking out of his back? You could make really cool shapes. May want to crop out a little to fit them in. I guess if the turtle was supposed to be a realistic concept of camouflage, then it wouldn't have crazy things sticking way out.
Maybe some other species of fish with a splash of bright color hanging around too. Just be careful of focal points.
Great job. Excellent use of color, lighting, texture. I have to agree with what Artfix says, though. The shell in particular could have been taken a little more into the coral and away from the usual turtle look.