well I am pretty sure that it is a Porcellanid Crab, i was checking some pics on line and this one seems to be what I can see, the color is alittle different, mine is a darker green color. Does anyone know about these things? I have had my tank for almost 2 months now and I just saw him yesterday for the first time. The pic I found online is this:
Porcellanid Crab photo - clippo photos at pbase.com