Greeting I have 9 large mussels all seem healthy and have moved them selves to different parts of a 40 gal tank all but one have dug themselves into the substrate and seem to be happy as a clam. One however has this stuff on it it is nowhere else in the tank and it open and closes like normal but somehow I feel some thing is up with it can any one tell me what it is and if I should be doing something about it