If it not closing up or open the mantels in the light it in a dying process, it not looking good the way you describle it. You need to do a better homework when you buy clams next time. Happen to me also! good luck, clams are quite sentive to light and water parameter, Make sure your tank water have enought Calcium to support clams mininum 400ppm. Good luck! oh I don't know how large it your tank you want to remove it before it starting to die and give out all the slimmy stuff in your tank