Those crabs are typically non-destructive communal species. If they look a bit furry and have blue eyes, watch them closely as they can cause some damage to the sps they are in.
Re: lenses
Actually, inverting the macro lens won't really work. The normal procedure is to take a wide open lens, such as a 35 or 50mm and invert that over the top of the longest lens you have. For example, you place a 200mm on the camera, and then put an inverted 50 over the top of it, you effectively get a 4:1 magnification.
The procedure works well in theory, but for practical purposes it is a huge pain. Your focal plane is
tiny! It basically requires an adapter (easy to get for $10-15 on ebay) that matches the threads for each lens. In reality, you need a tripod and a mechanism for moving the camera back and forth (just changing focus doesn't do it). That said, you can't do this in-tank for coral pictures. The focal plane, while tiny, is also very close to the camera.
For closeup I'd recommend getting some of the diopters (+1 to +5 in various combinations). Those will work in the tank, although they also limit the focal distance of the lens, and they will do really heavy magnification if you have the patience to learn to use them.
Is that all clear as mud now??
Wade