I could never get my maxima to attach to the rocks either. The trick is finding a place that it likes, which means low current and high light. Sometimes using little pieces of rock to support it helps prevent it from falling, but if it keeps jumping, then it doesn't like the spot. You can always keep moving it until it finds a place. I mentioned the lighting because the clam seems really black in the photo. A black clam (with some specific exceptions) is a bad sign and usually is the result of an overcompensation of zooxanthallea to get more energy. If the black is actually blue, then thats not an issue. You seem to have plenty of light. I'm trying to go through the list of potential trouble makers. If there is definite growth, then the clam is healthy. Do you feed it anything? If the clam is growing and healthy, then a parasite seems a likely alternative to stress. Have you added any new fish or other critters to the tank recently? What other inverts do you have? Also, describe the crabs you saw. Finally, check under the mantle at the lip of the clam to see if there are any unwelcome guests at the top.
