The "scute" right below the mantle should remain stark white(sometimes hard to see if hidden by the mantle). If it is anything but stark white, it could mean the clam is on it's way out.
FWIW, I've kept my maxima on the sand under a 6 bulb ATI T5 setup with no issue. It has actually grown 3 inches in the 2 years I've had it. I've also read that aquacultured maximas often loose their ability to attach. If they have no need to attach when young, they often loose this ability. So don't be concerned if it doesn't attach to anything. Mine sits on the sandbed and only attaches to the fine grains of sand.
http://animal-world.com/Aquarium-Cor...s/Maxima-Clams